gistfile1.txt

String trackName = '4BYGxv4rxSNcTgT3DsFB9o' //replace this with a call to get the track ID

final List<String> tracksToAdd = Arrays.asList("spotify:track:"+trackName);

// Index starts at 0
final int insertIndex = 1;

final AddTrackToPlaylistRequest request = api.addTracksToPlaylist("thelinmichael", "5ieJqeLJjjI8iJWaxeBLuK", tracksToAdd)
  .position(insertIndex)
  .build();

try {
  request.get();
} catch (Exception e) {
   System.out.println("Something went wrong!" + e.getMessage());
}