NOTE:
Oops! I didn't notice that @DawnC had already answered this question. I saw that right after I posted.
Sorry for the duplicate.
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Are you referring to a Box D control number? If so, it's not required by the IRS. It's a number used by your payroll department. 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...
