@Andi **********
it was a letter addressed to "whom it may concern". My dentist put my passport number on there, and a bunch of detail about the treatments I'm due to have and how it's important that I adhere to the scheduled treatments to ensure the best outcome.
I asked my dentist to put this level of detail on there because I wanted to ensure that I didn't get rejected. But as mentioned above, I've heard that a simple appointment card may be enough to get approved. The extra detail doesn't hurt though!