Set Field Value on a list? in a workflow? Field > Static Value > Selection. In SuiteScript, you would need to isolate the ID, so I imagine a formula would be the same.

@TwoStep Yes, set field on a custom field of type list/record (projects). And yes, in a workflow. I know how to do it for a static value or from another field of the same type. But I'm trying to create a dynamic selection based off another field (that's not the same list/type). Trying to understand what (if anything) I can output in the formula to get it to select the correct record. Could script, but that's always my last resort. Workflows are much more stable and low-maintenance IMO.