How can I make a custom field optional for issue creation and required from that point on?

[Robert Horan]

It seems to me that the field would have to be either required or optional at all times.

[Veera]

@Robert Horan 

You should be able to do this using the Workflow Validators.

You may need to link a screen with the required fields to the workflow transition and validate if the user entered the value for the field using the workflow validator.