update assignee to specific user if custom field value selected first value

[Astala Vista]

By using script runner and groovy how can I update assignee based on custom field 'first value(select list)

[Gezim Shehu]

def cf = customFieldManager.getCustomFieldByName("Your Custom Field")
def cfValue = cf.getValue(issue)

if(cfValue == 'your desired value'){
   issue.setAssignee('username')
}