mitmproxy-modify-request.py

import json 

def modify(content):
    content["Injected"] = "Luidna"
    return content

def request(flow):
    if flow.request and flow.request.content:
        content = json.loads(flow.request.content)
        new_content = modify(content)
        flow.request.content = json.dumps(new_content).encode()

test.sh

#!/bin/bash

mitmproxy -s change-request.py --listen-port 8081 --mode reverse:http://localhost:55002