I don’t understand how I get a project code for form SCH IN-OCC

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 would 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.