Are you referring to a Box D control number on a W-2? A control number is not needed by the IRS. It's used for the employer's or payroll provider's purpose, or sometimes to import.
You could try just leaving it blank. If a blank Box D produces an error, you can try making up a number in the format shown in this FAQ:
https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/box-control-num...
