You should reach out to your previous employer. If you withdrew from their profit-sharing program, your distribution should be reported to you on a 1099-R, or possibly a 1099-Misc.
If your employer is not issuing a 1099 for some reason, you can report the income under Wages & Income > Miscellaneous Income 1099-A, 1099-C > Other Reportable Income.
Enter the amount and source/explanation.
Here's more info on Not Filing a 1099.
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