http.client returning 502 bad gateway error

Question

I'm trying to send a post request to https://trades.roblox.com/v1/trades/send using proxies. These proxies use user:pass authorization. However, I get a 503 bad gateway error when sending the post request.

Here is my code:

import base64

proxies = [] #where my proxies are, they are inserted as IP:port.



username = 'myusername'

password = 'mypassword'
cred = '{}:{}'.format(username, password)

cred = base64.b64encode(cred.encode('utf-8')).decode('utf-8')

conn = http.client.HTTPConnection(random.choice(proxies))
conn.set_tunnel(
      "https://trades.roblox.com", port = 443, headers = {'Proxy-Authorization': 'Basic 
 {}'.format(cred)}
)

conn.request(
    "POST",
    "/v1/trades/send",
    data,
    {'content-type': 'application/json'}
)

print(conn.getresponse())

If anyone knows why this error occurs that would help me a lot, thanks.