Copy Custom Text Field contents to Incident Subject field

Summary

How to copy Custom Text Field contents to out of the box Incident Subject field

Content

Our agents in Agent console are not using the out of the box Incident Subject field which is hidden. They are using a custom field (Data type : Text Area which allows more than 4000 characters so they can put more details instead of the 256 character subject field). 

How do I copy the first 256 characters of the custom field to Incident.Subject field?

I looked at Workflows and business rule but did not find any solution?

Version

18D