How can I change an optonal field into a required field in a workflow.
For example if an issue is set on "in work", an optional field should be set on required.
Thank you very much for answering my question.
You need to add a validator to the transitions which going out from the status "in work". You can read more here
Also you need to install Jira suite utilities plugin
If I have understood your question correctly, You will need to add a field required validator on the transition which takes the issue to "in work". This is provided by a free add-on JIRA suite utilities which I believe is a must have add-on for every JIRA instance (Probably you are already using it)
Refer the below KB ,
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