You have to go to a government designated testing center at no cost to you. Most likely, hopefully. they will tell you where to go, etc. when you enter or the hotel will have the information?
Did the I/O mention a possible home visit to you? During the under consideration period it is possible they will visit the home to ensure the marriage is legit and may require two Thai witnesses to sign a paper attesting that they know you and to their knowledge the marriage is legit.