Form 3921 is not a form that must be reported on your tax return (like a Form W-2). It does have important information that might need to be reported (or used to calculate information that is required to be reported). The exercise of Incentive Stock Options (if you don't sell) is not a taxable event for "regular" tax. It does create taxable income for Alternative Minimum Tax (AMT). The "bargain element" of your ISO exercise is the difference between the exercise price and the fair market value a the time of exercise. On your Form 3921, that would be the amount in box 3, minus the amount in box 4, multiplied by box 5. If the amount is significant, it can trigger alternative minimum tax. In that case, your stock has two different cost basis. If the bargain element from your ISO exercise is minimal, you don't need to report anything. You do need to maintain a copy of the form 3921 for your records. If you want to enter it, type "Alternative Minimum Tax" in search, then click Enter. This will take you to the AMT section.
For more information, see this TurboTax article: Incentive Stock Options
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