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