This being Thailand, expect different reactions at different registration offices. Patience of Jesus may be required.
When I bought my car, it was already registered up country, 10 hours drive away. Initially I was told I would have to go there first, then return to Bangkok and register again. I had visions of driving all the way to Nakhon Nowhere only to be sent back empty handed.
I refused to go!
I just sat there at Nonthaburi reg' office till they gave in. In the end a nice Thai lady came out to help. She said it was her last day at the reg' office and she had always hated the stupid rules. It made her day to break the rule. It all took a few hours, but Roadrunner left clutching a new blue book and new reg plates.
They will want your passport and proof of address, plus the car and its papers.