Call Command from Page.cs

Hi everyone

i have Login page with Button

private async Task ClickAsync(Button signinbutton)
{
signinbutton.IsEnabled = false;
RegisterButton.IsEnabled = false;
try
{
string HtmlText = GetHtmlFromUri("http://google.com");
if (HtmlText == "")
{
await DisplayAlert("خطأ", "لايوجد اتصال بالانترنت , يرجى المحاولة لاحقاً.", "حسناً");
}
else if (!HtmlText.Contains("schema.org/WebPage"))
{
//Redirecting since the beginning of googles html contains that
//phrase and it was not found
await DisplayAlert("خطأ", "لايوجد اتصال بالانترنت , يرجى المحاولة لاحقاً.", "حسناً");

            }
            else
            {
                UserDialogs.Instance.ShowLoading("");

                if (UsernameEntry.Text != null && PasswordEntry.Text != null)
                {
                    var AccessToken = await accountApi.Login(UsernameEntry.Text, PasswordEntry.Text);

                    if (AccessToken != null)
                    {
                        Settings.AccessToken = AccessToken;
                        UserDialogs.Instance.HideLoading();

                    ****    /// Here i want to call Command Login From ViewModel****
                    }
                    else
                    {
                        UserDialogs.Instance.HideLoading();
                        await DisplayAlert("تنبيه!!.", "خطأ في تسجيل الدخول", "حسنأ");
                    }

                }
                else
                {
                    UserDialogs.Instance.HideLoading();
                    await DisplayAlert("تنبيه!!", "فضلاً تأكد من الحقول", "حسنأ");
                }
            }
        }
        catch (Exception e)
        {
            await DisplayAlert("يرجى المحاولة لاحقاً.", e.Message.ToString(), "حسناً");
        }
        finally
        {
            UserDialogs.Instance.HideLoading();
            signinbutton.IsEnabled = true;
            RegisterButton.IsEnabled = true;
        }
    }

My View Model

public class SignInViewModel : BaseViewModel, ISignInViewModel
{
private INavigationService navigationService;
private string _email;
private string _error;
private string _password;

    public SignInViewModel()
    {
        var canLogin = this.WhenAny(x => x.Email, x => x.Password,
            (e, p) => !string.IsNullOrEmpty(e.Value) && !string.IsNullOrEmpty(p.Value));

        GoRegister = ReactiveCommand.CreateFromTask(async () =>
           await navigationService.NavigateAsync(typeof(ISignUpViewModel)));

        Login = ReactiveCommand.CreateFromTask(async () =>
            {
                await navigationService.NavigateToMainPage(typeof(IMainViewModel));
            }, canLogin);

    }


    public string Error
    {
        get => _error;
        set => this.RaiseAndSetIfChanged(ref _error, value);
    }

    public ICommand GoRegister { get; }


    public ICommand Login { get; } // **Calll This Command **


    public string Email
    {
        get => _email;
        set => this.RaiseAndSetIfChanged(ref _email, value);
    }

    public string Password
    {
        get => _password;
        set => this.RaiseAndSetIfChanged(ref _password, value);
    }

    public override string Title => "تسجيل دخول";
}

I tried execute() but not worked

my Goal is to navigation to another View Using navigation service

Any Help Please

Thanks in Advance