Email referral source when Deal enters new stage

I used Zapier for years to set up various workflows. My new company finally cleared us to integrate our email to PD. I'm trying to use the internal workflow automation to send an email to the Buyers Agent's email (custom field) when a Deal moves from one stage to the next. When I choose the Template for this and go through the steps, I come to the To field, I can only choose five options, all pertaining to the Owner. What am I doing wrong, does anyone have a similar Workflow you can screenshot please?