Basically I am looking to add a validation for a custom field while editing the Jira ticket.
I can achieve the same while creating Jira through workflow validation.
I need to know how it can be done while Editing?
Technically, there still isn't a way to do it really. The standard trick is what you've already suggested - move the edit into the workflow
However, if you have the Script Runner add-on, that has some "edit validation" hooks you can use. I'm not sure if there are other things that can get to these hooks yet.
They've been removed from SR, as they were too limited in what they could do.
They could catch ordinary edits, and bulk changes, but if you changed issues in Jira Software, Portfolio or Service Desk, the structure of Jira's code meant there was no way to put validators in.
Behaviours might be able to help you a bit though, that's worth a look.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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