import socket
HOST="sso.yunhaixingtong.com"; PORT=80
body="loginname=AAA&pwd=BBB&captcha="
req=(
    "POST /api/login HTTP/1.1\r\n"
    f"Host: {HOST}\r\n"
    "Content-Type: application/x-www-form-urlencoded\r\n"
    f"Content-Length: {len(body)}\r\n"
    "Connection: close\r\n"
    "\r\n"
    + body
)
s=socket.create_connection((HOST,PORT),timeout=20)
s.sendall(req.encode())
data=b""
while True:
    chunk=s.recv(4096)
    if not chunk: break
    data+=chunk
s.close()
text=data.decode('utf-8','ignore')
# print status line + headers (up to first blank line)
head=text.split("\r\n\r\n",1)[0]
print("=== RAW RESPONSE HEAD (native form POST) ===")
print(head[:1500])