So, you entered Thailand on visa-exempt. You are going to go to IT Square Laksi Plaza temporary office, not Chaengwattana. It's located just next to Lak Si station on SRT Dark Red Line.
You will go here.
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Both IT Square Laksi office and Chaengwattana office accept walk-ins without pre-booking no problem.
Here is general information for tourism extension. Bangkok usually doesn't ask for TM30 receipt for tourism extension, but you have to provide the address inside Bangkok.