To redirect requests without the www to www on your website, add the following rules to your web.config.
Make sure your.domain.name below is changed to your domain name.
NOTE: If you already have a web.config with rules configured, add the rule below, otherwise a full web.config example is below it.
Rule
=========================================================================
<rule name="Redirect to WWW" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTP_HOST}" pattern="^your.domain.name$" />
</conditions>
<action type="Redirect" url="http://www.your.domain.name/{R:0}" redirectType="Permanent" />
</rule>
Full Web.Config with Rule
==========================================================================
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="Redirect to WWW" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTP_HOST}" pattern="^your.domain.name$" />
</conditions>
<action type="Redirect" url="http://www.your.domain.name/{R:0}" redirectType="Permanent" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>