As I understand your question, you are a UN staff member, and these are wages. If you have an Annual Statement of Earnings, you are a staff member, not "self-employed" and the income should be reported on line 7 as wages. The UN prefers that you not use Schedule C, because it causes problems with the tax reimbursement system.
You will use the W-2 entry screen to enter your full salary, but then you will indicate that you didn’t get a W-2 and link to a Form 4852. Turbo Tax will ask you to enter an explanation, which you can do in a few sentences: The UN doesn't issue W-2s, so you used your Statement of Earnings, etc.
Because you are a US citizen, you need to indicate that these earnings are subject to self-employment tax. Use the Schedule SE Adjustments Worksheet and enter your UN salary on Part II, line 4 "Other SE income reported as income on Form 1040, Line 7."
The problem with this answer and approach is that you won't be able to e-file. It will be rejected because you won't have an EIN to enter in the W-2 screen, because the UN doesn't have one, and you can't use any workarounds anymore (like entering all x's or all 1's or all 9's in place of an actual EIN).
TurboTax seems to be either unaware of this problem, or refuses to deal with it. The least they could do would be to (I) understand the problem experienced by their customers in this area and then (ii) either deal with it, or give accurate and complete information about why they can't and the consequences for us.
