Make chained functions wait for each other to execute

How do I make a chained function wait for the function before it, to execute properly?

I have this excerpt from my module:

var ParentFunction = function(){
    this.userAgent = "SomeAgent";
    return this;
}

ParentFunction.prototype.login = function(){
    var _this = this;

        request.post(
            url, {
            "headers": {
                "User-Agent": _this.userAgent
            }
        }, function(err, response, body){
            return _this;
        })
    }
}

ParentFunction.prototype.user = function(username){
    this.username = username;
    return this;
}

module.exports = parentFunction;

And this is from within my server:

var pF = require("./myModule.js"),
    parentFunction = new pF();

parentFunction.login().user("Mobilpadde").exec(function(data){
    res.json(data);
});

The problem is, that the user-function won't wait for login to finish (Meaning, it executes before the login returns _this). So how do I make it wait?