Exercise of ISO is not taxable for regular tax but subject to AMT. You will enter the bargain element as taxable income for AMT under Taxes > Alternative Minimum Tax (6251).
Make sure you keep track of this as you'd need to adjust the AMT basis for capital gain/loss in the year of disposition.
In the old days, ISO often triggers AMT, which would give rise to MTC in future years but this is less likely now after TCJA.
Still an AllStar