Go to partner hub. just do a search for "ethics and compliance" I think it should be the first hit.

Then follow the instructions and fill out the form.

You are theoretically anonymous in the process, however you will be asked who else has knowledge of the situation, and a for a detailed description of your complaint. any people you mention by name may get a phone call about it, and potentially their direct supervisors as well. So if you want to stay anonymous try to write it in a way that doesn't specifically identify you.

About 3 weeks after I made a complaint my DM sent out a memo basically telling SMs to stop what I had complained about. No info on any action taken about the specific person, but it was made clear that the conduct I reported was not acceptable.