To achieve this follow these steps:
1. Insert the web form on to a web page in HTML form. Do not insert it as a module.
2. Use CSS styles to hide (not remove) the Username field. You can do this by wrapping it in a HTML div element which is set to hidden. The code to hide the Username field is provided below:
<div style="display:none;"><input type="text" name="Username" id="Username"></div>
3. Add the following code to make the email address field automatically update the Username field when an email address is entered:
<input type=text name="EmailAddress" onBlur="document.getElementById('Username').value=this.value;">