Check out example codes for "external font family uwp c#". It will help you in understanding the concepts better.

Code Example 1

FontFamily="[FontFilePath]#[FontName]"

Code Example 2

FontFamily="Assets/Fonts/ArimaMadurai-Black.ttf#Arima Madurai"
//or
FontFamily="Assets/Fonts/ArimaMadurai-Black.ttf#Arima Madurai Black"

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