On your T2042 form, at the top Part 1, there is a question about accounting method, you must check cash or accrual in order for the amount to populate on line 9941, which comes from INVADJ form.
So after about 5 hours talking to TurboTax support it seems to work if you enter your Optional Inventory Adjustment using EasyStep. It then populates line 9941 but you'll have to also enter the amount on the INVADJ(2) form.
More info on entering Optional Inventory Adjustment on line 9941:
1.) You only have to enter minimal data to step through EasyStep.
2.) Be sure to select you participate in Agri-Stability to get to the T1163 form.
3.) Enter your Optional Inventory Adjustment in EasyStep, then SAVE your return.
4.) Now you can go to Forms and your Optional Inventory Adjustment should be on line 9941.
5.) I can now change my Optional Inventory Adjustment on the INVADJ form using the Forms method.
I hope this saves other users hours of time talking to TurboTax support.