You cannot personally contribute to a solo 401(k) because you are not self-employed. The S corp must make the deposits and report them on the S corp's tax return. The elective deferral must be made from wages that would otherwise be shown in box 1 of your W-2 and the elective deferral would then be shown with code D in box 12 of your W-2. You were required to make your election for the elective deferral by December 31 and the S corp would need to have deposited the elective deferral withheld from your wages, generally by January 15 under Department of Labor regulations. Depending on the timing, you might be able to reimburse the S corp for the amount of the employer contribution and the S corp might be able to amend it's tax return to report the employer contribution, but the $18,000 that was intended as an elective deferral is probably an excess contribution subject to penalty.
