The starting point for advising this client should be reading this 35-page pdf publication from the UN:

https://www.un.org/Depts/oppba/accounts/tax/pubs/sticlatest.pdf 

Some of its more significant content:

“United Nations emoluments are considered wages or earnings from employment, and must be reported as such on line 1 of IRS form 1040, “Wages, salaries, tips, etc.”. United Nations earnings are not business income and should not be reported on IRS schedule C.

“United States citizens working for the United Nations and serving in the United States are taxed on United Nations earnings for Social Security contribution purposes as if they were self-employed. Their Social Security contribution on the gross amount of United Nations taxable income for 2019 must be calculated using IRS schedule SE (self-employment tax). This does not imply, however, that United Nations emoluments should be reported as business income on IRS schedule C.”

I wouldn’t get too hung up on trying to pound this square peg into the round hole of IRS electronic filing.