title: Creating a custom website audience and prefilling with past data
body: <p>We'd like to create a custom audience, based on website pixel data. This should be prefilled with past visitors that match the criteria</p>
  
  <p>Here's an example which does this, but does not prefil</p>
  
  <pre><code>$set = new CustomAudience(null, $account_id);
  $set-&gt;setData(array(
  CustomAudienceFields::ID,
  CustomAudienceFields::NAME =&gt;"Test audience",
  CustomAudienceFields::DESCRIPTION =&gt;"Test audience from API",
    CustomAudienceFields::SUBTYPE =&gt;'WEBSITE',
   CustomAudienceFields::RULE =&gt; '{ 
    "event": {
      "i_contains": "ViewEvent" 
    }
  }',
  CustomAudienceFields::RETENTION_DAYS =&gt;90
  
  
  ));
  
  $set-&gt;create();
  </code></pre>
  
  <p>What needs to be passed through to trigger the prefil? The docs specify a 'prefil' parameter for curl calls but I cant see the relevant field in the PHP SDK?</p>