Would Box 15 be the same thing as Employer ID number? Like it is in box b?

You can use the federal ID number that is listed in box "b" if there is not state ID number listed on your W2 in box "15".

In some instances, the state mailing abbreviation also works (that is, the 2 letters specific to your state e.g. FL, NY, CA etc.)

Hope This Helps!!