Since you retired and you discount on ESPP stock was not reported on a W-2, you can add this amount as Other Income, Not Reported on a W-2 to your return.
In the Income section, scroll all the way down the topic listings to Other Miscellaneous Income, 1099-A, 1099-C.
On the next page, choose Add/Update, and say YES to 'did you receive any other wages?'. Choose Reason Code H on the screen where you enter Employer and Amount.
The income will show on Form 1040, Line 1g. You will be subject to SS and Medicare tax on the income, as if it had been reported on a W-2, on Form 8919.
Here's more info on Employee Stock Purchase Plans.
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