title: Facebook API returns 'Unsupported post request' when adding users to Custom Audience
body: <p>When I try to add users to a Custom Audience (using a POST to endpoint <code>/{custom_audience_id}/users</code>, with parameters <code>users</code> and <code>hash_type</code>), it gives me  </p>
  
  <p>{'error': {'message': 'Unsupported post request.', 'code': 100, 'type': 'GraphMethodException'}}  </p>
  
  <p>This message has popped up for other people on Stack Overflow when they try to do things for which they did not get the right Facebook permissions, but I do have ads_management permissions for the user (the user is actually my own account, so I have all the permissions). I have Ads API Access, and the message at the top of (<a href="https://developers.facebook.com/docs/reference/ads-api/custom-audience-api" rel="nofollow">https://developers.facebook.com/docs/reference/ads-api/custom-audience-api</a>) says that even though Custom Audience API Access is different from Ads API Access, if you have Ads API Access then you also have Custom Audience API Access. And Ads API Access is not the issue (I can create Ad Campaigns, Ad Sets, Custom Audiences, etc.).  </p>
  
  <p>In conclusion, I have ads_management permission and Ads API Access, and can do a bunch of Ads stuff already, except when I try to add users to a Custom Audience, I get the above JSON-formatted error. Any guidance on why, or at least how to go about figuring out why?  </p>
  
  <p>(and the Facebook error code explanation for 100, "Invalid parameter", is largely no help, as are the vague error message and error type)</p>