You will have to use some script for that.
Clicking the href, the value of username should be retrieved trought script, and be passed along with the href-url.
What don't you just let the user post his data, and just give an error, telling that the username exists? You just show the same page again with the same data, but with an error on top. You could even go further and suggest a username...
You will have to write some script for that. You can attach an onClick event to the <a> tag and pass the value of the username to that funuction.
In the fucntion make call to the servlet that validates the availabilty of the usename. As olivier suggested you can you can even suggest the user the avilable user names.