Thank you Kurt! So I have my module set up like this, but I get this error. Is this an access error? "[422] APIModel V20_0 does not support field null (Program)"
You're likely getting that error as you have the record set set to First Matching Record and it needs to be All Matching Records. You want the scenario to pull all program records that match your search criteria, since none have that field populated yet. Here's example below of how you can set up your search. Notice under collections I toggle the map and use projects:referenceNumber as you wanted to get all the reference numbers of projects underneath your program. You'll have some other things to think about in your scenario, such as, what if there are no projects yet under the program? what do you want your scenario to do in this case as it can't populate the custom field yet. You will also need to use the iterator function under the Flow module to help list all the projects under the program to get their IDs and reference numbers.
