WPF How to use a FactoryMethod for Custom Controls

Question

.NET 4.7.2

I have a simple UserControl containing a custom TextBox. I am trying to only define the interface in Xaml and create the instance of the custom TextBox by specifying a FactoryMethod.

<UserControl x:Class="MyProject.TestView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"             
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:MyProject"
         mc:Ignorable="d" 
         d:DesignHeight="450" d:DesignWidth="800">

<Grid>
    <local:IInterfaceTextBox x:FactoryMethod="local:Factories.CreateInterfaceTextBox" x:Arguments="{x:Null}"  Text="{Binding Text}" CustomText="{Binding CustomText}"   />
</Grid>

The Factories class:

namespace MyProject
{
    public class Factories
    {
        public static IInterfaceTextBox CreateInterfaceTextBox() => new InterfaceTextBox();
    }
}

First Error: The property "FactoryMethod" does not exist in the "http://schemas.microsoft.com/winfx/2006/xaml" namespace.

When I set xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" the error message says UserControl can not be found in http://schemas.microsoft.com/winfx/2009/xaml

Does anyone know how to solve this?

Just in case you are wondering, why I'm trying to this in the first place... I'm curios about new things :-)