How do I parse application/vnd.surveymonkey.response.v1+json? I recently asked a question about setting up SurveyMonkey webhooks here: https://drupal.stackexchange.com/questions/252571/surveymonkey-v3-webhook-callback

After laborious troubleshooting, I've determined that the issue is that I need to be able to parse the request as application/vnd.surveymonkey.response.v1+json. 

How do I do that?

There's no option to add this type of parser, or a custom parser, in the REST server config. Googling for a few hours has come up with nothing. In a Drupal REST endpoint, how can I use this parser type?
So the answer is to use hook_rest_server_request_parsers_alter(). This way, the application/vnd.surveymonkey.response.v1 Content-Type can be mapped to the standard JSON parser.

function MY_MODULE_rest_server_request_parsers_alter(&amp;$parsers) {
  $parsers['application/vnd.surveymonkey.response.v1+json'] =  'ServicesParserJSON';
}