well, if I were them and you told me you're newly self-employed and you have no invoices yet, and from the previous company you can't provide me any documents... I'd clearly refuse your application.
If you're a freelancer without your company, I believe you should show them some of your agreements with the clients - whatever you think is necessary to prove that you actually work like that and that you do it legally
They just want to see if you can legally work remotely from Thailand. So it's either some confirmation from your CEO, or confirmation from yourself if you're the CEO. Some invoices or agreements will be a plus, but are not required. Also it can be necessary to show them that the company works legally, so some documents from the gov institutions stating that the company has been registered.
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