If you really entered Thailand on a visa-exempt scheme, not a visa waiver based on bilateral agreement, You are going to go to IT Square Laksi Plaza temporary office, just next to Lak Si station of SRT Dark Red Line.
Better to have TM30 Receipt. But if you are only staying in Bangkok this time and without an extensive past history in/to Thailand, reservation/booking may be used for address proof, but cannot be guaranteed.
You can book online here
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