Just show the $1,600 as a capital contribution. Not a loan as you won't be repaid.
I assume that you are a cash method taxpayer, so there is some exposure to claiming the amount paid in 2018 on the 2017 tax return. I would not lose sleep over this and claim it in 2017 knowing that there is some exposure.
As a shareholder in an S corporation, you should be maintaining a basis schedule. Once this is updated for your final K-1, you can now determine your overall gain or loss on this investment.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
