Few people if any are told to go take an antigen test when you show up at Chaengwattana. The target people who have been out of bangkok in the last two weeks mostly.
Just show up, fill out the form, if they tell you to test there's a testing site right there.
OR
you can get an antigen test from
****
before you go, test and take a photo of it (with the time/date stamp) and they'll accept that as proof.
Most people just fill out the form, and get right in.