1) Decide where, if anywhere, you want it to appear on your tax return.

2) Enter it as if you had no particular document but needed a number to appear there.

https://www.federalregister.gov/documents/2018/10/10/2018-20304/guidance-related-to-section-951a-glo... is a reasonable plain-English explanation, to the extent that any of that stuff is reasonable or explainable.