Change owner field based on a dynamic user lookup on the form

Hi All,

I'm trying to change the owner field of a custom entity record to the same value in another user lookup on the form (set owner field value as resource).

I've tried to create a WF that assign records but it seems I can't select my custom user lookup ("Resource") in the properties:

This needs to happen on create and it's not like one of two records that I can go and change the values by hand.

Does anyone know how to achieve this functionality?

Thanks,

- Bod