I can't convince CustomGPT to make HTTP Request with text/plain in body

GPT builders
1

I have the following simplified OpenAPI file:

{
  "openapi": "3.1.0",
  "info": {
    "title": "Ping pong call",
    "description": "Send a message and receive it back",
    "version": "v1.0.0"
  },
  "servers": [
    {
      "url": "<url>"
    }
  ],
  "paths": {
    "/execute": {
      "post": {
        "description": "Send a message and receive it back",
        "operationId": "runCode",
        "requestBody": {
          "required": true,
          "description": "The message that you want to receive back.",
          "content": {
            "text/plain": {
              "schema": {
                "type": "string"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "Message reiceived back."
          }
        }
      }
    }
  },
  "components": {
    "schemas": {}
  }
}

I am unable to make this work. I think that ChatGPT only wants JSON in the body. Has anyone encountered this issue? Am I doing something wrong?