Just report it as other miscellaneous taxable income. There is no need to enter a fake 1099-R with a fake tax number.
Separately, there are some situations where foreign retirement benefits that are considered similar to mandatory Social Security in the US might be subject to tax treaty provisions that affect where and how much tax you pay. If you want to discuss that, you will need to tell us about the country and the benefit, your citizenship and residency status, and I will locate an expert to assist.
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