When entering the detail for the K-1 form, there will be several screens to enter data. Eventually, you will come to a screen where it asks if there are amounts in any boxes 4-17 and you would want to check Box 12. The next screen should have a drop-down menu for the type of deduction code. You will be able to select "S" and then enter the amount. Sample screenshots are attached.
Also, please note that K-1 forms for different entities have different uses for box 12. So, make sure that you are selecting the right type of entity in TurboTax for the K-1 form you have received. For instance, box 12 on a Partnership/LLC K-1 form is for a Section 179 deduction and it will not have a drop-down menu. Box 12 on a SCorp K-1 is for Other Deductions and will have a drop-down menu. The entity form type will be noted in the upper left-hand corner under the words Schedule K-1. Sample 1065 K-1 is attached.
