If you received a Schedule IN K-1 that reported the credit, you can find the code there. Otherwise, you should have received a credit certification letter from the authorizing entity with the relevant Certification/Project Number information.
To claim the credit you're entering on Schedule IN-OCC, you should have requested and received certification to claim the credit from the appropriate authority. If so, you will have received a certification letter from the authorizing entity (or on your IN K-1, if applicable.)
See the Instructions for Other Certified Credits on the back of the schedule linked above for more information.
