Parse.com / Javascript - Save users objectid string as user pointer

How do I save a user pointer to an object when i have the object id of the user.

I am able to save the object to the class in Parse but the assignee is always 'Undefined' in Parse.

e.g. I have retrieved the user object and can get the username / object id etc through:

function getUserFromUsername(username) {

Parse.initialize("...", "...");

console.log('The username passed in is: ' + username);

var User = Parse.Object.extend("_User");
var query = new Parse.Query(User);
query.equalTo("username", username);

query.first({
     success : function(result) {
        // Do something with the returned Parse.Object values
            var userPointer = new Parse.User();
            userPointer = result;
            console.log(userPointer.get('username')); // this returns the correct username
            return userPointer;

    },
    error : function(error) {
        alert("Error: " + error.code + " " + error.message);
    }
});
}

Which is called from my save task function below: (Note, I've logged all relevant fields and they return as expected.

function saveNewTask(clientName, taskTitle, taskDue, assigneeArray) {

Parse.initialize("...", "...");

var x;
for (x in assigneeArray) {

    var Task = Parse.Object.extend("Tasks");
    var task = new Task();

    task.set("title", taskTitle);
    task.set("date", taskDue);

    var thisAssignee = GetUserFromUsername(assigneeArray[x]);
    task.set('assignee', thisAssignee);

    task.save(null, {
        success : function(task) {
            // Execute any logic that should take place after the object is saved.
            console.log('New object created with objectId: ' + task.id);
        },
        error : function(gameScore, error) {
            // Execute any logic that should take place if the save fails.
            // error is a Parse.Error with an error code and message.
            console.log('Failed to create new object, with error code: ' + error.message);
        }
    });
}

}