American in Germany - W9

I’m a US citizen living in Germany, thus a German tax resident. While signing up, I am asked to enter my German tax id number, and to sign a W-8BEN, which states “Do not use this if you are a US citizen,” and where, as a US citizen, I am directed to instead use a W9. Also I’m asked to certify that I am not a US person. How can I sign up and submit the correct forms?