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.

• Click the Federal tab.
• Click Wages & Income.
• Select "I'll Choose What to Work On."
• Under "Less Common Income," click "Miscellaneous Income, 1099-A, 1099-C."
• On the next screen, click "Other reportable income."
• The next screen asks, "Any Other Taxable Income?" Click Yes.
• On the next screen, "Other Taxable Income," enter "Manufacturer incentive payment" as the description. Enter the amount and click Continue.
• On the next screen click Done.

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:

1. Click on Federal > Wages & Income.
2. Scroll down to the Other Common Income section. 
3. Click on the Start/Revisit box next Form 1099-MISC. 
4. On the screen Did you get a 1099-MISC?   Yes 
5. Enter the information from the 1099-MISC.
    
6. On the following screens, answer the following questions:
   • Did it involve work that's like your main job? No.
   • You got it in 2020
   • No, it didn't involve an intent to earn money

You would still characterize it as enter "Manufacturer incentive payment" as the description.