您可以按照MAUI文档中的描述为此编写一个行为。
在此行为中，您为ViewModel的绑定声明了一个附加属性，并订阅条目的Unfocused事件以更新附加属性。

public class EntryLostFocusBehavior : Behavior<Entry>
{
    public static readonly BindableProperty TextProperty = BindableProperty.CreateAttached(
        "Text",
        typeof( string ),
        typeof( EntryLostFocusBehavior ),
        null,
        BindingMode.TwoWay,
        propertyChanged: TextPropertyChanged );

    public static string GetText( Entry entry ) => (string)entry.GetValue( TextProperty );
    public static void SetText( Entry entry, string value ) => entry.SetValue( TextProperty, value );

    private static void TextPropertyChanged( BindableObject bindable, object oldValue, object newValue )
    {
        var entry = (Entry)bindable;
        entry.Placeholder = newValue as string;
    }

    protected override void OnAttachedTo( Entry bindable )
    {
        base.OnAttachedTo( bindable );
        bindable.Unfocused += Entry_Unfocused;
    }

    protected override void OnDetachingFrom( Entry bindable )
    {
        base.OnDetachingFrom( bindable );
        bindable.Unfocused -= Entry_Unfocused;
    }

    private void Entry_Unfocused( object sender, FocusEventArgs e )
    {
        var entry = (Entry)sender;
        SetText( entry, entry.Text );
    }
}

在你的视图中使用这个

<Page ...
      xmlns:b="clr-namespace:MyMauiApp.Behaviors"
      ...>

  <Entry b:EntryLostFocusBehavior.Text="{Binding Value1}">
      <Entry.Behaviors>
          <b:EntryLostFocusBehavior />
      <Entry.Behaviors>
  </Entry>

  <Entry b:EntryLostFocusBehavior.Text="{Binding Value2}">
      <Entry.Behaviors>
          <b:EntryLostFocusBehavior />
      <Entry.Behaviors>
  </Entry>

</Page>

这就是视图/视图模型。没有代码隐藏，没有其他属性。