In the link below, you can add tracks to a library playlist https://developer.apple.com/documentation/applemusicapi/add_tracks_to_library_playlist .. Is there a musickit JS API endpoint to add a music video to a playlist?
Apologies, answered my own question so will post answer for anyone in future.
Yes the endpoint https://api.music.apple.com/v1/me/library/playlists/{id}/tracks (https://developer.apple.com/documentation/applemusicapi/add_tracks_to_library_playlist) requires a LibraryPlaylistTracksRequest object.
The LibraryPlaylistTracksRequest object has a LibraryPlaylistRequestTrack object (https://developer.apple.com/documentation/applemusicapi/libraryplaylistrequesttrack).
The 'type' property/attribute on this object can be songs, music-videos, library-songs, or library-music-videos. (https://developer.apple.com/documentation/applemusicapi/libraryplaylistrequesttrack)