You can enter the SPIFF in TurboTax by entering it as "other income" so that it will be reported on Schedule 1, line 21, with a description that you create TurboTax versions vary, but you should be able to navigate using these basic instructions.
Do you have updated steps for this? This doesn't seem to still work in 2021
@Tateman
You can enter the 1099-MISC in TurboTax as "Other Common Income," which is reported on line 8 of 1040 Schedule 1. Please use the following steps:
You would still characterize it as enter "Manufacturer incentive payment" as the description.