You are talking about eExtension for pre-approvement. Unless you have a very specific reason to use eExtension, you can just go to the Bangkok Immigration temporary office at IT Square Laksi to apply for a 30 days tourism extension by yourself. Online booking slots are available a few days after Songkran, or they accept walk-ins any day.
You can book online at IT Square Laksi here
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If you have to use eExtension, you need to contact that official service provider for detail.