If you add controls from third party libraries in a C# WPF project, the XAML file will normally have lines like this one.
This will perhaps not work with FsXaml.
The designer and the compiler accepts that line, but there will probably be an exception at runtime complaining about the 3rd party type not being found when reading the XAML.
Try something like the following instead.
This then is an example of a control that depends on the above.
<xctk:IntegerUpDown Name="tbInput" Increment="1" Maximum="10" Minimum="0" Canvas.Left="13" Canvas.Top="27" Width="270"/>
The library used in this example is the Extended Wpf Toolkit, available free of charge through NuGet or as installer. If you download libraries through NuGet, then the controls are not available in the Toolbox, but they still show in the designer if you add them manually in XAML, and the properties are available in the Properties pane.