To balance out my S Corp return for a PPP already forgiven in 2020:

Sch M1:

enter amount of PPP on line 1 (it's included in "book" income)

enter on line 5a same amount as above (tax exempt)

M1 balances

Sch M2:

line 3 other additions in the Other AA column

that should get your M1 & M2 all in balanced/sync with you Retained earnings on the balance sheet 

Finally - show it as a contribution (stock) on your basis info worksheet 

and it shows up on the shareholders basis statement giving them basis without appearing on the k-1

That's how I'm doing it !