Writing a listener that creates a parent link to issue created

[Kristin Rains]

I need help writing a listener script using the Scriptrunner. 

We have a custom field called Client Project Category and it is a single issue picker. I want to set a listener that when that field is populated the Client Project Category issue should also be set as the parent of the issue being created. 

Example: I am creating a new Task (ABC-1) and in that task I set the Client Project Category field as (XYZ-2). When ABC-1 is created I also want XYZ-2 to be set as the Parent Of ABC-1. 

The Client Project Category filed could be populated when creating an issue or it could be populated after creation by editing.