[Includes] Add Validator
To add a validator:
Log into your Jira Server instance as an Administrator.
Click on the Settings icon in the upper right corner.
Select Issues.
In the left-hand panel, click Workflows.
Click Actions ( ) for the workflow you want to edit and select Edit.
Edit the Transition:
When viewing the Workflow in Diagram view, select the Transition and click the Validators link. Click Add validator at the top of the list of existing validators.
When viewing the Workflow in Text view, click the name of the Transition then select the Validators tab. Click Add validator at the top of the list of existing validators.