I don't think it's possible to impersonate the MS Form submitter to post a message on a Teams channel via PA. You technically could post as someone else if you use their connection in the Teams action inside the flow, but it will still not be dynamic, and the same static user will post for the Form submissions of all the others.
Does your use case support a change in this requirement? Would it be Ok to have the name of the Form submitter include in the message body, for example?