EntryCompleteViewModel

using System.ComponentModel;
using System.Runtime.CompilerServices;
using Xamarin.Forms;

namespace EntryCompleteEvent
{
    public class MyViewModel : INotifyPropertyChanged
    {
        public MyViewModel()
        {
            IsControlVisible = true;
            CompletedCommand = new Command(_ => CompletedCommandExecuted(_));
        }


        private bool _isControlVisible;
        public bool IsControlVisible
        {
            get { return _isControlVisible; }
            set
            {
                _isControlVisible = value;
                OnPropertyChanged();
            }
        }

        public System.Windows.Input.ICommand CompletedCommand { get; protected set; }
        private void CompletedCommandExecuted(object param)
        {
            IsControlVisible = false;
        }


        public event PropertyChangedEventHandler PropertyChanged;
        protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

EntryCompleteXaml.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:local="clr-namespace:EntryCompleteEvent"
             x:Class="EntryCompleteEvent.EntryCompleteEventPage">
    <StackLayout>
        <Entry Placeholder="Email Address"
               Keyboard="Email"
               x:Name="EmailEntry"
               IsVisible="{Binding IsControlVisible}">
            <Entry.Behaviors>
                <local:EntryCompletedBehavior Command="{Binding CompletedCommand}" />
            </Entry.Behaviors>
        </Entry>
    </StackLayout>
</ContentPage>

EventCompleteBehavior

using System;
using System.Reflection;
using System.Windows.Input;
using Xamarin.Forms;

namespace EntryCompleteEvent
{
    public class EntryCompletedBehavior : Behavior<Entry>
    {
        protected override void OnAttachedTo(Entry bindable)
        {
            base.OnAttachedTo(bindable);

            AssociatedObject = bindable;

            bindable.BindingContextChanged += Bindable_BindingContextChanged;
            bindable.Completed += Bindable_Completed;
        }
        protected override void OnDetachingFrom(Entry bindable)
        {
            base.OnDetachingFrom(bindable);

            bindable.BindingContextChanged -= Bindable_BindingContextChanged;
            bindable.Completed -= Bindable_Completed;

            AssociatedObject = null;
        }

        private void Bindable_BindingContextChanged(object sender, System.EventArgs e)
        {
            OnBindingContextChanged();
        }
        private void Bindable_Completed(object sender, System.EventArgs e)
        {
            if (Command == null)return;

            //object parameter = Converter.Convert(e, typeof(object), null, null);
            if (Command.CanExecute(e))
            {
                Command.Execute(e);
            }

        }

        protected override void OnBindingContextChanged()
        {
            base.OnBindingContextChanged();
            BindingContext = AssociatedObject.BindingContext;
        }

        public Entry AssociatedObject { get; private set; }
        public static readonly BindableProperty CommandProperty =
                BindableProperty.Create("Command", typeof(ICommand), typeof(EntryCompletedBehavior), null);
        public ICommand Command
        {
            get { return (ICommand)GetValue(CommandProperty); }
            set { SetValue(CommandProperty, value); }
        }
    }
}