title: UrlRewrite condition based on custom HTTP header
body: <p>I'm trying to configure a rule in UrlRewrite that has 2 conditions:</p>
  
  <ul>
  <li>HTTP header <code>HTTP_HOST</code> needs to match a certain domain (api.contoso.com)</li>
  <li>A custom HTTP header <code>x-app-version</code> needs to be present in the HTTP request</li>
  </ul>
  
  <p>based on this info, I'd like to redirect to a different version of the API in the back-end.</p>
  
  <p><strong>Problem</strong>
  The rule is behaving as expected on the first condition. 
  It kicks in when <code>HTTP_HOST</code> matches api.contoso.com.
  However, it ignores my custom <code>x-app-version</code> header.</p>
  
  <p><strong>Assumption</strong>
  So, I fear that a UrlRewrite condition only can be defined in combination with the limited set of predefined HTTP headers from the dropdown (<code>HTTP_HOST</code>, <code>REMOTE_ADDRESS</code>, <code>REMOTE_HOST</code>, &hellip;)</p>
  
  <p><strong>Question</strong>
  Is assumption correct or should this be possible whatsoever? 
  Is there a mistake in my config or other approach to have conditions based on a custom defined HTTP header?</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;rule name="ARR_test2" enabled="false"&gt;
    &lt;match url="(.*)" /&gt;  
    &lt;conditions&gt;
      &lt;add input="{HTTP_HOST}" pattern="api.contoso.com" /&gt;
      &lt;add input="{x-app-version}" pattern="1" /&gt;
    &lt;/conditions&gt;
    &lt;action type="Rewrite" url="https://FARM_apiv1/{R:0}" /&gt;
  &lt;/rule&gt;
  </code></pre>