C# UWP MediaPlayerElement customize TransportControls CCSelection with Choose Subtitle

Question

In C# UWP i overrided the MediaPlayerElement's trasnportcontrol. I didn't find anything about how can i extend my code to add a button inside the CCSelectionButton's MenuFlyoutItem in order to be able to add subtitles to the video after it has been loaded. Just like in the Media player:

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I used this documentation to add subtitle to the current videos, but i didn't find anything about my problem mentioned above: https://github.com/Microsoft/Windows-universal-samples/blob/e13cf5dca497ad661706d150a154830666913be4/Samples/VideoPlayback/cs/Scenario4_DisplayingOutOfBandClosedCaptions.xaml.cs