In the
<system.webserver></system.webserver>
section of your web.config file add a rewrite rule like this:
<rewrite>
<rules>
<rule name="mydomain" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTP_HOST}" pattern="^mydomain.com$" />
</conditions>
<action type="Redirect" url="http://www.mydomain.com/{R:0}" redirectType="Permanent" />
</rule>
</rules>
</rewrite>