Hi {{first_name}},
I'm interviewing for a [[type of role]] position at [[company]], and your perspective would carry the most weight for this one. The role focuses on [[key responsibility or skill area]], which maps directly to the work we did together on [[specific project or initiative]].
If you're willing, I'd prepare you with a short brief covering the role, what to expect on the call, and the specific points I'd want highlighted. No call would happen without advance notice from me.
Would that work for you?
[[Your name]]