✅ How do I specify custom base class for user control?

I declared a custom base class derived from
UserControl
. Now in a second assembly, I need to derive from that. The base class doesn't provide a XAML page. Instead, the derived class would. So
MyCtrl
is derived from
MyCtrlBase
which in turn derives from
UserControl
. How do I specify that in the XAML? I tried the following, but it didn't work. Seems the designer doesn't see my custom base class.
<Base:MyCtrlBase
    x:Class="BestChat.Prefs.Ctrls.Global"
    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:BestChat.Prefs.Ctrls"
    xmlns:Base="clr-namespace:BestChat.Prefs.Data;assembly=BestChat.Prefs.Data"
    mc:Ignorable="d">
    <Grid></Grid>

Can someone tell me what I need to change?
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