How to get WM_COPYDATA in python

Question

i'm trying to do (what should be) some minor work with win32. I basically just need to send a message and wait for a response.

1. I need the HWND or window handle of my python script/program to actually send the message.
2. I need to receive a WM_COPYDATA message from the end program.

I'm able to find the HWND of my python program with a script provided by ColonelFai1ure#0706 on the python discord server:

#gets the HWND of the active window
def getActiveWinID():
    print(ctypes.windll.user32.GetForegroundWindow())
    return ctypes.windll.user32.GetForegroundWindow()

Although I have to keep my PyCharm instance as the active window, it works fine, i'm able to send a message with the HWND now!

So I send a message with:

import ctypes
import subprocess # can be async
from ctypes import wintypes
from ctypes import *

with subprocess.Popen(r"C:\Users\lafft\Downloads\2021-04-11\3.35\ProxyTool\ProxyAPI.exe "
                     r"-changeproxy/US/NY/'New York' -citynolimit -proxyport=5001 -hwnd=330320",
                     stdin=subprocess.PIPE, stdout=subprocess.PIPE) as process:
    for line in process.stdout:
        print(line)

It works! The end program receives my message and performs the work I need it to do, this is the output:

b'Api executed successfully.\r\n'

Now I need to receive WM_COPYDATA from the end program to my hwnd, I modified some py2 code I found here:

class ACOPYDATASTRUCT(Structure):
    _fields_ = [
        ('dwData', c_ulong),
        ('cbData', c_ulong),
        ('lpData', c_void_p)
    ]
PCOPYDATASTRUCT = POINTER(ACOPYDATASTRUCT)


class Listener:

    def __init__(self):
        message_map = {
            win32con.WM_COPYDATA: self.OnCopyData
        }
        wc = win32gui.WNDCLASS()
        wc.lpfnWndProc = message_map
        wc.lpszClassName = 'MyWindowClass'
        hinst = wc.hInstance = win32api.GetModuleHandle(None)
        classAtom = win32gui.RegisterClass(wc)
        self.hwnd = 330320
        print(self.hwnd)

    def OnCopyData(self, hwnd, msg, wparam, lparam):
        print(hwnd)
        print(msg)
        print(wparam)
        print(lparam)
        pCDS = cast(lparam, PCOPYDATASTRUCT)
        print(pCDS.contents.dwData)
        print(pCDS.contents.cbData)
        status = wstring_at(pCDS.contents.lpData)
        print(status)
        win32gui.PostQuitMessage(0)
        return 1

l = Listener()

win32gui.PumpMessages()

Unfortunately, the message never comes in. I'm using PyCharm to run this, if it makes a difference.

I have 0 idea what to do now, and tutorials on the internet for this is sparse or written in python 2. Any help is appreciated, a link to a tutorial, some code, anything.