how to set validation message for the required field.

[Veeravenkata Sampathkumar Garapati]

This question is in reference to Atlassian Documentation: Specifying field behavior

how to set validation message for the required field.

[Nic Brough -Adaptavist-]

For a mandatory field done in field configurations, a red star will appear automatically.

If you're using a validator, then that can return a proper validation message for a field when the check fails, but they can't put anything on the front end - you'll need to put it in the description or use one of the add-ons that can add messages.