Class

    class User : INotifyPropertyChanged {

        private string _username;
        public string username {
            get { return _username; }
            set {
                if (_username != value) {
                    _username = value;
                    RaisePropertyChanged();
                }
            }
        }


        private string _password;
        public string password {
            get { return _password; }
            set {
                if (_password != value) {
                    _password = value;
                    RaisePropertyChanged();
                }
            }
        }


        #region Notify Property Changed Members
        public event PropertyChangedEventHandler PropertyChanged;
        private void RaisePropertyChanged([CallerMemberName] string propertyName = "") {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion
    }
}

viewModel

    class RegisterViewModel {

        public RegisterCommand RegisterCommand { get; set; }
        User User;

        public RegisterViewModel(User user) {
            RegisterCommand = new RegisterCommand(this);

        }

        internal void SendData(object data) {
            Application.Current.MainPage.Navigation.PushAsync(new Page1());
        }
    }
}

Register Command

   class RegisterCommand : ICommand {

        public RegisterViewModel RegisterViewModel { get; set; }

        public RegisterCommand(RegisterViewModel registerViewModel) {
            RegisterViewModel = registerViewModel;
        }

        public event EventHandler CanExecuteChanged;

        public bool CanExecute(object parameter) {
            if (parameter != null) {
                return true;
            }
            return false;
        }

        public void Execute(object parameter) {
            RegisterViewModel.SendData(parameter);
        }
    }
}

MainPge.xaml

 <ContentPage.Resources>
        <model:User x:Key="model" />
        <vm:RegisterViewModel x:Key="RegisteVM" />
    </ContentPage.Resources>

    <StackLayout>
        <Entry Placeholder="Username"
               Text="{Binding username, Source={StaticResource model}}" />
        <Entry Placeholder="Password"
               Text="{Binding password, Source={StaticResource model}}"/>
        <Button BackgroundColor="DarkCyan" Command="{Binding RegisterCommand, Source={StaticResource RegisteVM}}" >
            <Button.CommandParameter>
                <model:User/>
            </Button.CommandParameter>
        </Button>
    </StackLayout>

SecondPage

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             x:Class="Question.View.Page1">
    <ContentPage.Content>
        <StackLayout>
            <Label Text="Welcome to Xamarin.Forms!"
                VerticalOptions="CenterAndExpand"
                HorizontalOptions="CenterAndExpand" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>