save picker options from collectionview to database

I created a list of attendance in xaml with 3 tables.
1. The first table is the date that I put in the picker.
2. The second table is the name of the students that I put in the collectionview along with a picker containing 4 options.
3. The third table is attendance, which is stored in the database.

this is an example of c# ..

using AdmKelas.Models;
using System;
using System.ComponentModel;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;
using System.Collections.Generic;
using System.Linq;

namespace AdmKelas.Halaman
{
    [DesignTimeVisible(false)]
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class EditAbsen : ContentPage
    {

        public EditAbsen()
        {
            InitializeComponent();
        }
        protected override async void OnAppearing()
        {
            List<Siswa> sis = await App.Dtsekolah.GetSiswaAsync();
            daftarSiswa.ItemsSource = sis;
            List<Kalpen> kal = await App.Dtsekolah.GetKalpenAsync();
            tglKalpen.ItemsSource = kal;
            base.OnAppearing();
        }
        async void SimpanData(object sender, EventArgs e)
        {
            var absen = (Absen)BindingContext;
            await DisplayAlert("Sukses!", "Data tersimpan", "Ok");
            await App.Dtsekolah.SaveAbsenAsync(absen);
            await Navigation.PopAsync();
        }
        async void HapusData(object sender, EventArgs e)
        {
            var result = await DisplayAlert("Peringatan!", "Data absen ini akan dihapus?", "Ya", "Batal");

            if (result == true)
            {
                var absen = (Absen)BindingContext;
                await App.Dtsekolah.DeleteAbsenAsync(absen);
                await Navigation.PopAsync();
            }
            else
            { return; }
        }
    }
}

code xaml.

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
             xmlns:abstractions="clr-namespace:ImageCircle.Forms.Plugin.Abstractions;assembly=ImageCircle.Forms.Plugin" 
             xmlns:local="clr-namespace:AdmKelas.CS"
             NavigationPage.HasNavigationBar="False"
             x:Class="AdmKelas.Halaman.EditAbsen">
    <ContentPage.Content>
        <StackLayout>
            <Grid Margin="20,20,20,0" HeightRequest="120">
                <!--<Entry x:Name="en1" IsVisible="false" Text="{Binding HariAbsen}"/> SelectedIndexChanged="Pick"-->
                <Frame Style="{StaticResource FrameBox}">
                    <local:CustomPicker Style="{StaticResource pickr}" 
                                        Title="Tanggal"
                                        x:Name="tglKalpen"
                                        SelectedItem="{Binding HariAbsen}"
                                        ItemDisplayBinding="{Binding Tanggal, StringFormat='{0:dddd, dd MMMM yyyy}'}" 
                                        HorizontalOptions="Center">
                    </local:CustomPicker>
                </Frame>
                <Label Text=" calendar-alt Tanggal " Style="{StaticResource LabelFrame}" HorizontalOptions="Center" FontSize="16"/>
            </Grid>
            <CollectionView x:Name="daftarSiswa" 
                            Margin="20,0,20,20"
                            SelectionMode="None"
                            ItemsLayout="VerticalGrid,2"
                            HorizontalOptions="StartAndExpand"
                            VerticalOptions="CenterAndExpand"
                            EmptyView="Belum ada data yang ditampilkan. Anda harus memasukan data terlebih dahulu dengan mengabsen siswa sesuai tanggal.">
                <CollectionView.ItemTemplate>
                    <DataTemplate>
                        <StackLayout>
                            <BoxView BackgroundColor="Black" Opacity="0.1" HeightRequest="1"/>
                            <Grid ColumnDefinitions="0.7*,*" Padding="6">
                                <Grid Grid.Column="0">
                                    <BoxView HeightRequest="1" HorizontalOptions="StartAndExpand" WidthRequest="1"
                                             Color="Goldenrod"/>
                                    <Grid HorizontalOptions="CenterAndExpand">
                                        <abstractions:CircleImage Source="{Binding FotoSiswa}"
                                                                  Aspect="AspectFill"
                                                                  HeightRequest="60"
                                                                  WidthRequest="60"
                                                                  BorderColor="Goldenrod"
                                                                  BorderThickness="1"
                                                                  Margin="6" />
                                        <Label Text="{Binding NamaKecil}"
                                               BackgroundColor="Black"
                                               VerticalOptions="Center"
                                               HorizontalOptions="EndAndExpand"
                                               TextColor="gold"
                                               FontSize="12"
                                               Padding="5,0,5,0"
                                               FontAttributes="Bold"
                                               Margin="10,0,0,-50"/>
                                    </Grid>
                                </Grid>
                                <Grid Grid.Column="1" HorizontalOptions="CenterAndExpand">
                                    <local:CustomPicker Style="{StaticResource pickr}" Title="Absen" SelectedItem="{Binding CekList}">
                                        <local:CustomPicker.ItemsSource>
                                            <x:Array Type="{x:Type x:String}">
                                                <x:String>Hadir</x:String>
                                                <x:String>Sakit</x:String>
                                                <x:String>Izin</x:String>
                                                <x:String>Alpa</x:String>
                                            </x:Array>
                                        </local:CustomPicker.ItemsSource>
                                    </local:CustomPicker>
                                    <!--<RadioButton Text="Hadir" />
                                    <RadioButton Text="Sakit"/>
                                    <RadioButton Text="Izin"/>
                                    <RadioButton Text="Alpa"/>-->
                                </Grid>
                            </Grid>
                        </StackLayout>
                    </DataTemplate>
                </CollectionView.ItemTemplate>
            </CollectionView>
            <BoxView BackgroundColor="Transparent" HeightRequest="1" Margin="-10" 
                     HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"/>
            <AbsoluteLayout>
                <StackLayout AbsoluteLayout.LayoutBounds="0,1,1,60"
                             AbsoluteLayout.LayoutFlags="XProportional,YProportional,WidthProportional">
                    <StackLayout Orientation="Horizontal" 
                                 Padding="8"
                                 BackgroundColor="#2196F3">
                        <Button Style="{StaticResource butn}" Text="times-circle"  BackgroundColor="#EC2B4F" TextColor="AliceBlue" Clicked="HapusData" />
                        <Button Style="{StaticResource butn}" Text="check-circle" BackgroundColor="#7DDA49" TextColor="#0A402C" Clicked="SimpanData"/>
                    </StackLayout>
                </StackLayout>
            </AbsoluteLayout>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

how to write code behind the xaml to enter data into the third table?