How do I enter the amount for WV Motor Vehicle Tax Adjustment on my state tax form line 21 (A) if I wasn't prompted?

The Motor Vehicle Tax Adjustment portion of the West Virginia return has loaded in TurboTax Online since you posed your question, so you should be able to enter your information now.  To do so: 

1. Open TurboTax Online
2. Select  Tax Home in the left pane
3. In the left pane, select the dropdown to the right of State Taxes
4. Select  Review/Edit
5. Page through any screens until you get to Status of your state returns
6. Select  Edit next to West Virginia
7. Proceed through the state interview until you get to the Motor Vehicle Property Tax Adjustment Credit screen
8. You'll be asked whether you received an MV-1 schedule from the West Virginia Tax Division.  If you answer yes, the next screen will let you enter your MV-1 data.      But when you answer no, you bypass the chance to enter that information.

See Motor Vehicle Property Tax Adjustment Credit on West Virginia's Income Tax Credits for Property Taxes Paid page for more information.

If the TurboTax Online navigation suggested above doesn't look like what you're seeing, see the TurboTax Help article Why does the TurboTax navigation menu look different for me?.

You can't look at the actual form in TurboTax Online until you're ready to file.  See the TurboTax Help article How do I view and delete forms in TurboTax Online? for more guidance.