As a retiree, I receive monthly payments from my United Nations' pension. Don't get a 1099-R. What should I answer on Box 7?

As the United Nations Pension Fund doesn't send me a 1099-R form, if I try to input the information on the section for 1099-R, I can't go further as I don't know what to answer on box 7. Also, UNPF doesn't inform me which is the taxable amount. Please, help.

Thanks!

Eleana