From the 1040-NR instructions:
Enter 85% of the total amount from
box 5 of all of your Forms SSA-1042S
and Forms RRB-1042S in the
appropriate column of line 8 of
Schedule NEC. Attach a copy of each
Form SSA-1042S and RRB-1042S to
the front of Form 1040-NR.
Line 25g—Form(s) 1042-S
Enter on line 25g the total amount
shown as federal income tax withheld
under chapter 3 or 4 on your Form(s)
1042-S. The withholding credit should
be shown in box 10 of your Form(s)
1042-S. Attach Form(s) 1042-S to the
front of your return.
Be sure to attach to the front
of your return a copy of all
Form(s) W-2, 1042-S,
SSA-1042S, RRB-1042S, and
8288-A. Attach to the front of your
return Form(s) 1099-R if tax was
withheld
