1. Create a new project
2. Add the following custom element

 
{code:java}
[UxmlElement]
public partial class MyElement : VisualElement
{
[UxmlAttribute, CreateProperty]
public string MyString
{ get; set; }
[UxmlAttribute, CreateProperty]
public GameObject MyPrefab { get; set; }
}
{code}

3. Add the element in UI Builder

4. Right-click the custom fields, and notice that there is no Add Binding option

If you add a binding via UXML is will also not show in the builder.

If you change the attribute names to start with lowercase then it will work.