I finally figured this out. For those of you who couldn't get it to work with the suggestions above, try this:
1. Go to Wages & Income > Investment Income > Stocks, Cryptocurrency, Mutual Funds, Bonds, Other (1099-B)
2. Add the stocks you sold in 2021 that were a result of prior year ISO exercises
3. Under the Sales Info > What type of investment did you sell? Select "Incentive stock options (ISO)" (This doesn't make sense cause you're not technically selling ISOs. It's just bad copy on the UI <- feedback)
4. Click continue and select "none of the above applies"
5. It'll go through the "Next, we'll help you figure out your cost basis" flow
6. Add the # of shares you sold and fill out the 3921 form info (e.g. strike price, FMV, date of exercise, etc.)
7. Hit done and now Turbotax is able to connect the dots for your prior years exercise with the current tax year sale. You should see your AMT tax credit applied correctly now.
