@Micky ********
The hotel has to notify the immigration about your stay. Just go to the hotel and tell them you need a paper to prove, that you‘re staying there. Then take that to immigration. They will give you the certificate. Costs about 500 Baht and needs 2 passport pictures - not sure anymore about these details. We went to Bangkok Bank first, got a sample of that immigration letter, went there and asked for that, then the guy at the information counter gave as a form that the hotel had to fill in. Then back the next day and we got the Certificate. My father needed it and he was on visa exemption.