URL rewrite is not working as expected IIS7

Question

I am trying to achieve the following rewrite URL in IIS 7:

subdomain.domain.com/username should become subdomain.domain.com/?username

When I try to match the pattern /([^/]+) and test it with /username, it gives me

{R:0} /username
{R:1} username

and for the rewrite URL I have used /?{R:1}

However when I test it in a browser the site throws a 404 error and does not rewrite the URL.

Please what can I be doing wrong.