A W-8BEN is an IRS mandated form which all creators unrelated to the United States need to submit to confirm their foreign status. The U.S. requires Fansly to report all creators that earn more than $20,000 in a calendar year, and who are U.S. citizens or who have reason to believe are U.S. citizens. Since Fansly is a company operating within the United States, and creators are paid by the U.S. dollar, this would lead the IRS to believe foreign individuals are U.S. citizens. In other words, the form is to prove non-US citizenship when submitting taxpayer information.
You can submit your W-8BEN by navigating to your Creator Dashboard, expand the list by clicking More, then select Tax Documentation.
If you were a U.S. citizen living abroad (or alternately a foreign citizen living or operating within the United States), tax treaties would ensure that you were not taxed twice on the same income. For most people, this is not applicable, and only your home country (assuming you live where you are a citizen) will tax your income.