You can leave box d blank. You really don't need a control number, it's sometimes used by companies internally.
If you get errors when attempting to e-file with an empty Box D, just enter any number in this format: 5 digits, space, 5 digits (for example, 12345 67890).
On the other hand, if you're trying to import your W-2 with a blank Box D, click skip import and then manually enter your W-2. You won't be able to import using the 5 digit + space + 5 digit solution for e-filing.