Admin of Jira cloud can extract issue fields key and modify which one should be used in message format.
To extract and view issue fields key:
Step 1: Log in to JIRA as an administrator
Step 2: Select your Current Project from "Projects" dropdown on the top menu
Step 3: Expand the left menu, click on 
Step 4: At the left hand side, scroll down until Slack Integration option
Step 5: Click on Configure
Step 6: Click on Tools tab
Step 7: Enter issue key and click on Issue data
To modify issue fields in message format:
Step 8: Pick your expected key the click on Message format tab
Step 9: Modify message then click on Send Test to view the result which is already sent to configured channel
