I think you mean visa-exempt entry, which 51 countries are eligible for. Visa-waiver is a different program, based on a bilateral agreement, and only several nationalities are applied.
Now if you are in Bangkok and want to apply for 30days tourism extension on your visa-exempt stamp, you are going to go to IT Square Laksi Plaza temporary office, not Chaengwattana.
You can book online. It is not mandatory so you can just walk in, but the online queue maybe saves you time.
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