How to validate blank field using QTP 11.00?

[newtester01]

Hi,

I am testing a web based application using QTP 11.00.
The application has a login page.
The login page has a text box with a "submit" button.
The name of the textbox is "username".

Question:

How to validate a blank field in QTP for the scenario below?

Scenario:
1. In Login page, do not enter any value for "username", click submit button.

Expected output in application:
Validation error is displayed "Please enter a username".