Dependency Property not changing the binded Value (MVVM-Helper)

I just can’t find the missing piece. My UserControl doesn’t update the binded property. I’m creating a new UserControl in my code behind. The SetValue and GetValue method are getting called. If the viewModel is initializing some value, my UserControl gets it. But if I change the text in the Textbox, the viewmodel doesn’t hold this value.

My Code:

public partial class MyUserControl : UserControl, INotifyPropertyChanged
{

       public MyUserControl()
       {
           InitializeComponent();
       }

       public static readonly DependencyProperty CommentToAddProperty=
        DependencyProperty.Register(
         nameof(CommentToAdd),
         typeof(string),
         typeof(MyUserControl),
         new FrameworkPropertyMetadata(
          default(string),
          FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
          OnCommentChange));


    public string CommentToAdd
    {
        get => (string)GetValue(CommentToAddProperty);
        set => SetValue(CommentToAddProperty, value);
    }

    private static void OnCommentChange(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {

        var myUC = d as MyUserControl;
        var comment = e.NewValue as string;
        myUC.CommentToAdd = comment;
        myUC.RaisePropertyChanged(nameof(CommentToAdd));
}

public partial class SomeWindow: Window
{
   public SomeWindow()
   {
     InitializeComponent();
     _vm = new MyViewModel();
     DataContext = _vm;
     var userControl = new UserControls.MyUserControl
     {
        CommentToAdd = _vm.CommentToAdd,  
     }

     \\Add usercontrol to a grid
   }

   
}

public partial class MyViewModel : ObservableObject
{
   [ObservableProperty]
   private string _commentToAdd;
}
    

This is my XAML code in my UserControl

<TextBox
    Grid.Column="0"
    Text="{Binding Path= CommentToAdd, Mode=TwoWay, 
    RelativeSource={RelativeSource 
        Mode=FindAncestor, 
        AncestorType=local:MyUserControl},  
    UpdateSourceTrigger=PropertyChanged}"/>

>Solution :

The assignment

CommentToAdd = _vm.CommentToAdd

does not establish a data binding of the CommentToAdd property.

In order to create a Binding, you would have to write

userControl.SetBinding(
    MyUserControl.CommentToAddProperty,
    new Binding("CommentToAdd"));

or without relying on the assigned DataContext and on the fact that the property binds TwoWay by default

userControl.SetBinding(
    MyUserControl.CommentToAddProperty,
    new Binding
    {
        Source = _vm,
        Path = new PropertyPath("CommentToAdd"),
        Mode = BindingMode.TwoWay
    });

However, you should usually not be creating UI elements in code behind. Better declare the control in XAML:

<MyUserControl CommentToAdd="{Binding CommentToAdd}"/>