How to programmaticaly add value to a drop down list in RTC?

Hi,

I'm trying to programmatically add data to a custom workItem drop down list using a plugin.

Can I create a default value to store the value or does it have to be a value set?

How do I return the value so that it displays in the drop down list I created on the workItem?
I tried returning an Array, ArrayList and List, but I keep getting an error. Can the custom attribute be a default value?

Thanks!
Alicia