Assign a name to your run, and then create a FontFamily with a URI constructor :
Xaml:
<Run x:Name="MyTextRun">ABC</Run>
Code behind:
MyTextRun.FontFamily = new FontFamily(new Uri("/FontLibrary;Component/#Kootenay", UriKind.RelativeOrAbsolute), "Kootenay");
MyTextRun.FontSize = 36;
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