Redirect Non-WWW to WWW in Web.config (Except one spesific subdomain)

On my Windows 2012 server I have installed the URL Rewrite module in IIS. I have followed this guide to redirect non-www to www in web.config: http://www.surfingsuccess.com/asp/iis-url-rewrite.html#.VF6GBid0yAU

So far, so good!

The only problem is that we also host the subdomain "api.mysite.com". This API stops working when I apply the <rewrite> code in web.config. My question is: How can I change the <rewrite> code below to redirect non-www to www, except for the subdomain "api.mysite.com"?

    <rewrite>
      <rules>
        <rule name="RedirectToWWW" stopProcessing="true">
          <match url=".*" ignoreCase="true" />
          <conditions>
            <add input="{HTTP_HOST}" pattern="^mysite.com$" />
          </conditions>
          <action type="Redirect" url="http://www.mysite.com/{R:0}" redirectType="Permanent" />
        </rule>
      </rules>
    </rewrite>