How can I get the raw body from a REST endpoint in Hasura?

I'm trying to set up a webhook using Hasura's REST interface. I want to just accept any data that's sent to the webhook without validation.

Is it possible to do something like this:

REST endpoint

http://localhost:8080/api/rest/callback

Methods available

POST

GraphQL Request

mutation MyMutation ($rawBody: String!) {
  myMutation(payload: $rawBody) {
    mutationResult
  }
}