pick any districts that available on their system. I don’t think the Thai Embassy really care where you will stay in Thailand after you get your Non-O approval. It’s just make sure you have a reference address to stay in Thailand.
Udon Thani agent at Thai Visa Concierge service . They have Laos Border Run service if you needed help contact them. 063-747-0579 I used their services and everything was done smoothly.