Paste name when toggle

I have an element creation form which has field Users. Near this field I have button Choose to get names of users.

                        <StackLayout Orientation="Horizontal">
                        <Entry Text="{Binding Users,Mode=TwoWay}" 
                               IsEnabled="False"
                               HorizontalOptions="FillAndExpand"/>
                        <Button Text="Choose" 
                                Command="{Binding ChooseUserCommand}"
                                HorizontalOptions="End"/>
                        </StackLayout>

When I press on button Choose it will open new form with all users and item of switch near each of them.

                        <StackLayout VerticalOptions="CenterAndExpand">
                            <Label Text="{Binding User_Name}"  />
                        </StackLayout>
                        <Switch x:Name="SwitchPaste"  IsToggled="False" Grid.Column="1" />

I need to paste User_Name in field of Users when i press on switch (isToggled "True") and names should be separated by commas like ( Tomas; Alex; Bob and etc...) How I can do it?

I added image as example.