Found the answer here: specifically this:
Thanks this is what I needed. To give a more specific description for anyone else, I needed to check "enter them individually" after confirming it was an ISO sale. Then after checking the box that the cost basis is incorrect, I needed to check the field that said "I can't find it and need help" in regards to a supplemental form to the 1099-B. It seems this is because the broker I sold the stock with is not affiliated with the company that issues the original options... So they don't have any information other than the cost basis from when I transferred the options over.
This all led me to various prompts that allowed me to enter my 3921 form details from the ISO exercise. It appears this is doing what I need it to do.
Doing this allows you to fill out the necessary cost basis fix for AMT for each of your ISO transitions. Hope this helps others.
