You can use any method to let the taxpayer know the amount. There is no need to enter it on the K-1. Although you could put it on Line 17 with code AC, there is no need to do so.
The usual method is to put a notation in Box 14 of their W-2.
That amount was added to Box 1 of their W-2s, right?
What do you mean by "(where federal income tax was withheld)"?
