I need help specifying the proxy in the code below.

var _this = this;
  this.sfConn = new sf.Connection({
    loginUrl: 'https://login.salesforce.com'
  });
  console.log('About to log into salesforce');
  this.sfConn.login(username, password, function(error, userInfo) {
    if (error) { return console.log(error); }
    _this.userInfo = userInfo;
    callback(error, userInfo);
  });