That's fine, but that site is not authoritative on this question. The version of that same coupon code on National's site expires today:
It might get updated tomorrow (it used to expire at the end of May, so it has been updated before) but I suspect the new $20 coupon is replacing it.
Yes, that's probably an employer-specific code. Mine is also courtesy of my employer, it is also pre-populated, but it is a different number.