This is totally new to me as a tax preparer. Pass-thru entities use K-1s.
S-Corp only does 1125-E if gross receipts of $500k; I don't have any clients in that range so I have never HAD to furnish an 1125-E to the IRS.
However, the general guidance is to have officer compensation (if they are owner(s) of the S-Corp) to be reported on Schedule E, Supplemental Income.
That said, K-1 from an 1120S does not have "guaranteed payments" like a partnership's K-1. I typically let the $$ from the compensation flow to Schedule C since that auto generates the self-employment figures.
Obviously, it would be much, much easier if K-1s were standardized across entities,
