The part of the RMD that you made in 2019 will be reported on your 2019 tax return. It will be included in the 2019 Form 1099-R that you get next January. Even though it's part of your 2018 RMD, you report it in the year that you made it, not the year it is "for."

Your 2019 RMD will also be included in your 2019 Form 1099-R and your 2019 tax return.