I would like to put a validator at the time of issue creation, I want to validate that the requester is not the approver on the create screen before allowing the users to create the issue.
How can I put the validator ?
You will need to add this in the workflow for the create transition. Take a look at 3:26 on this video: https://youtu.be/RIKYX-z4meM
In your case you may need a scripting plugin such as jsu, jmwe or scriprunner that can compare that the reporter and approver are not the same user.
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