Python POST requests returns a "bad requests" when using a proxy

Question

I have been testing things out using this proxy to scrape some Yelp pages. Things have been great but I had to use it for a POST request but I kept getting a 400 status code (Bad request). Here is the thing if I try it without the proxy I get the expected results. Below is the code without using the proxy

import requests
import json


# Build a suitable JSON request for the required information
json_post = [
    {
        "operationName": "GetBusinessAttributes",
        "variables": {
            "BizEncId": "1IMivcKgsN8xUL5dFKQK3w"
        },
        "extensions": {
            "documentId": "35e0950cee1029aa00eef5180adb55af33a0217c64f379d778083eb4d1c805e7"
        }
    },
    {
        "operationName": "GetBizPageProperties",
        "variables": {
            "BizEncId": "1IMivcKgsN8xUL5dFKQK3w"
        },
        "extensions": {
            "documentId": "f06d155f02e55e7aadb01d6469e34d4bad301f14b6e0eba92a31e635694ebc21"
        }
    },
]
 
r = requests.post("https://www.yelp.com/gql/batch", json=json_post)

and here is the code with the proxy

import requests
import json


# Build a suitable JSON request for the required information
json_post = [
    {
        "operationName": "GetBusinessAttributes",
        "variables": {
            "BizEncId": "1IMivcKgsN8xUL5dFKQK3w"
        },
        "extensions": {
            "documentId": "35e0950cee1029aa00eef5180adb55af33a0217c64f379d778083eb4d1c805e7"
        }
    },
    {
        "operationName": "GetBizPageProperties",
        "variables": {
            "BizEncId": "1IMivcKgsN8xUL5dFKQK3w"
        },
        "extensions": {
            "documentId": "f06d155f02e55e7aadb01d6469e34d4bad301f14b6e0eba92a31e635694ebc21"
        }
    },
]
 
r = requests.post("http://api.proxiesapi.com/?auth_key=d325db3f85a49b0561aaa0dda5209b15_sr98766_ooPq87&url=https://www.yelp.com/gql/batch", json=json_post)

I've included the auth_key so you can try it out. I have a feeling the problem in how I am passing the json. Any help will be much appreciated.