You'll need to add that excess as Less Common income -> Miscellaneous Income, 1099-A, 1099-C -> Other Income not already reported on a Form W-2 or Form 1099. Answer Yes to receiving other wages, Yes to Any Other Earned Income, select Other, then enter the excess salary deferral. This amount will then appear on Form 1040 line 1h to make it taxable. This amount will be taxed a second time when eventually distributed from the 401(k) as a regular distribuiton.
