The comment should be entered on the transition screen (in the Comment section) of the transition. You can also customize the validation so that the members of certain groups are allowed to omit the comment.
To add 'Comment Required Validator' to a transition:
Click Edit for the workflow that has the transition you wish to configure the validator on.
In the Workflow Designer, select the transition.
Click on Validators in the properties panel.
Click on Addvalidator.
Select Comment Required Validator from the list of validators.
Click on Add to add the validator on the transition.
Input a message in the Error Message.
If you want to omit some groups, select the groups from the Available groups list and click on Add>> to add groups to the Allowed groups list.
Click on Add to add the validator to the transition.
When you add this validator to a transition and trigger the transition, the add-on checks for a value in the Comment field of the Transition screen. The comment field is suffixed with a red asterisk. If it is blank, the configured validation error message will be displayed. However, users from the selected groups can progress to the destination status without entering a comment.
Note that this validator does nothing if the transition is being made through SOAP, because the progressWorkflowAction method does not let you specify a comment.
Show asterisk (*) if the condition returns true upon showing the screen
By default, the red asterisk is not shown on the screen against the Comment field when using Conditional Validation, because the condition's value might depend on what the user will later enter on the transition screen. Check this option to show the asterisk if the condition returns true upon entering the transition or issue creation screen. This is useful when the condition is not dependent on the other fields shown on the screen (e.g. it only depends on the issue type, the current user's project role, etc.).