Your answer has confused me a bit. As far as I remember, there isn't a tax treaty form you need to fill out.
To sell under the "Collected by Itch" program, you must complete the tax interview. In it, you must provide your real information, and once it's validated, the percentage of sales tax withheld in the U.S. will be automatically determined based on your country of residence.
U.S. law requires a 30% withholding rate, but this percentage will be lower in the case of double taxation treaties between your country of residence and the U.S. You simply need to provide your accurate and truthful information in the tax interview that Itch requires you to complete, and that's all.