Make NO mention of yourself as a boyfriend. That's an immediate rejection. They want to be assured she has reason to return to Thailand and will actually return. Have a "friend" in the USA invite her. You can use a visa service such as Babel in Bangkok like I did.
When my fiancee applied for tourist visa we needed to have a friend invite her to USA with no mention of me. We were told they would deny the visa immediately if we did. We now have fiancee visa application pending. Takes forever. Got the tourist visa in like 3 months during covid. Been 13 months now waiting on fiancee visa.
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