I was getting mixed up; I say the word Will in the title and put it in the sentence. And read the translation as Willst du uns unterstützen? (So Wirst du uns unterstützen? does mean "Will you support us?")
The correct verb form is werden. I assume DL didn’t accept your version because the form werde does exist as well (ich werde), so it considered “Sie werde” a grammar error rather than a spelling mistake.