i need to populate a customfield which is user picker from scripted field

[acc1211]

So what i am basically trying to do is create a scripted field and write certain condition in that scripted field and when certain event for example edit happen on the issue this script runs and then it will allow the user based on the condition in the scripted field to send this value "the user " into the user picker . so now i want to be able to populate the user picker custom field from the scripted field with the user value when ever the condition is true .

Hope i didnt confuse .