No. you cannot just use all 9's. Depending on your situation, you may still need to print and mail your return. Did you receive a W-2? If so, when you get to the end of entering your W-2, on the next screen, where it says "Let's check for uncommon situations" you can check Employed by a foreign government or international organization"
Did you receive another document type like a 1099-NEC or no document at all? If so, then you can just enter your income in the business income section and not enter the EIN for the UN since they do not have one.
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