Could not understand the problem.
If I want to select the first preset (Radio x) just have to send:
http://ip:15081/favourites/6a0ead5dde684fa4b634ed379add4184?cmd=play
For any other preset, for example third one (Radio y) just have to send:
http://ip:15081/favourites/c4fc85c607d0405da33c7a9c2715a5e7?cmd=play
What do you mean with the long id and where do you have it from?