When the 1099-R comes in Jan of 2020 you will have to amend 2018 to report it. The excess must be added to income for the tax year that the excess was made in. You can just wait until next year if you prefer. Reporting it now is not required.
This will add the returned excess to your 2018 wages on line 1 exactly the same way that the 1099-R would. The only information that is sent to the IRS is the line 1 amount.
Excess 401(k) deferrals should be reported in:
(There are several screens to click through to get to the right place)
Miscellionious Income ->
Other Income not reported on a W-2 ->
Other wages (yes) ->
House Hold employee (Continue) ->
Sick Pay (Continue) ->
Other earned income (yes) (Includes excess salary deferrals)->
Source of income (other) ->
Any other income - enter the amount of the excess deferral and an explanation.
[Note: If there were any earning that were returned in 2019 then the earnings will be reported on a separate 2019 1099-R with a code 8 that goes on your 2019 tax return - do not enter the earnings here.]
For information see IRS Pub 525 page 10
<a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p525.pdf">https://www.irs.gov/pub/irs-pdf/p525.pdf</a>
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**