I want to say all you need is a booking. I bought a one way ticket to Singapore which was about $59 one way. They didn’t ask for it but I got it anyway to be on the safe side.
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.