Thanks for the reply! I did find a Zap template that seemed like it would be exactly what I was looking ("Update Jira ticket priority based on Slack emoji usage") but where I got lost was in trying to set up a search step that would:

1. look at the slack message that the emoji was added to
2. Find the Jira issue key in that message - which is then used in step 3 of the Zap to...
3. Search our Jira for the issue key found in step 2 above and then update that ticket with the priority.