You can just put in any combination of numbers in this specific format 5 digits, space, 5 digits (for example, 12345 67890).
That is just a number that the payroll team uses to identify your W-2 for their system purposes. Sometimes it may be blank since that is optional to them.
You can find more details on Box D and the Control Number here https://ttlc.intuit.com/questions/2669624
