I may suggest that you put more details and context in your question. It will help engagement. Please take a look at this guidance: How to Ask a Good Question
I am using Windchill 12.1. I am able to get the task assignee name in the workflow domain through WorkItem using the API "/WorkItem('{workItemId}')/CompletedBy" in Windchill.
But when I tried to get the task assignee name through the change notice number, I was not able to get it. I think, It may be because of differences in domains like 'PTC Change Management Domain' and 'PTC Workflow Domain'.