I think you are struggling to fill out Part II of the form. In this part, you have to add tax information. You can view the U.S.-Portugal tax treaty here. Article 13, which is about royalties, is of your use in this document. I have asked ChatGPT to fill out the form based on the U.S.-Portugal tax treaty and it returned the following results,
9.
“I certify that the beneficial owner is a resident of Portugal within the meaning of the income tax treaty between the United States and that country.”
10.
“The beneficial owner is claiming the provisions of Article 13, paragraph 2 of the treaty identified on line 9 above to claim a 10% rate of withholding on royalty income.”
You can further read the treaty document yourself as this is AI-generated. Thanks