Its a little tiny booth tucked in the corner, opposite from all the immigration officer desks. As soon as you turn the corner (towards immigration), you'll see it off by itself. You don't need to fill out or bring anything, just money. 😁
Recommend you use the Phuket Immigration branch (not Patong branch) if you are applying for the 60-day covid. When I did, i spoke with applicants who tried the get the Covid extension at Patong, but were told to go to the Phuket branch. Good luck!