Проблема решена, если кому интересно, код ниже. Всем спасибо.
from telethon import TelegramClient, events
api_id =
api_hash =
client = TelegramClient('user', api_id, api_hash, system_version="4.16.30-vxCUSTOM").start()
message = "Hello! Thank you for contacting me. \nI'll be back soon and reply to your message."
@client.on(events.NewMessage(incoming=True, func=lambda e: e.is_private))
async def handler(event):
sender = await event.get_input_sender()
await client.send_message(sender, message)
client.run_until_disconnected()
