You're supposed to report that as wages.  Agree it should theoretically go to Line 1h on the new F.1040, except PTO has not programmed an input to that line yet, AFAIK.  Line 1a is still where you should report that in PTO.  Reporting the income as something other than wages may impede your client's ability to claim tax reimbursement, as per the UN's policy.

The conundrum is that you will need an EIN to e-file.  But the UN doesn't include that on their Statement of Taxable Earnings.  It was suggested in a previous multi-year thread that the employee can approach the UN for the EIN to use.  That's not conclusive though:

https://accountants.intuit.com/community/proseries-tax-discussions/discussion/united-nation-does-not...

According to guidelines provided by the UN, staff is supposed to be able to e-file - that should mean that there is an EIN staff can use.

Would suggest that you have your client confirm the availability of an EIN with the UN.  It would be great if you can come back and update the group on what you find out.

Push comes to shove, you can try ignoring the critical diagnostic and see whether the return can be e-filed since there's no withholding to match up.  What's the worst that can happen, right?  You can always paper file if nothing works.