You can either leave it blank or just enter any number in this format: 5 digits, space, 5 digits (for example, 12345 67890). The IRS doesn't care what's in Box D because its use by payroll departments is optional.
A Control Number (Box D) is used by many payroll departments to uniquely identify a W-2 in their system. If your W-2 doesn't have one, it's no big deal – you can simply leave it blank.
Now, if you're trying to import your W-2 and you're persistently being asked to enter something in Box D, click skip import to get past this screen and manually enter your W-2 (which should only take a few minutes anyway).