Add a www to requests to the apex domain in web.config transforms

add-www-prefix.web.config.xml

 <system.webServer>
    <rewrite xdt:Transform="Insert">
      <rules>
        <rule name="Add WWW prefix to requests to apex domain" >
          <match url="(.*)" ignoreCase="true" />
          <conditions>
            <add input="{HTTP_HOST}" pattern="^thedomain\.com" />
          </conditions>
          <action type="Redirect" url="https://www.thedomain.com/{R:1}" redirectType="Permanent" />
        </rule>
      </rules>
    </rewrite>
  </system.webServer>