This is an old threads https://accountants-community.intuit.com/questions/240871-self-prepared-1120
https://accountants-community.intuit.com/questions/208317-marking-return-as-self-prepared
I don't use ProSeries, so I can not verify.

Conceptually, I have trouble with a Corporation doing it's own return. You are not the Corporation. You are likely an employee of the Corporation and are being paid, in part, for doing the tax return.