Change assigned from (user picker) based in a function clone issue and links

[grojas]

I would like to know how is possible change the assignee of an cloned issue in a post function using the value from a (user picker) custom field that is in a different source issued

the basic sintax to clone the same value from other issue is:

def cf = customFieldManager.getCustomFieldObjects(sourceIssue).find {it.name == 'QA Responsible'}
issue.setCustomFieldValue(cf, sourceIssue.getCustomFieldValue(cf))

i'm try to add something like this

def cf = customFieldManager.getCustomFieldObjects(sourceIssue).find {it.name == 'QA Responsible'}

issue.setAssignee(sourceIssue.getCustomfieldValue(cf))

but is not working!