ERR_NGROK_SHIP_POOL_JOIN_KEY_NOT_ALLOWED: The 'join_key' field must not start with 'cp_' - ngrok documentation

What does ERR_NGROK_SHIP_POOL_JOIN_KEY_NOT_ALLOWED mean?

ngrok returns ERR_NGROK_SHIP_POOL_JOIN_KEY_NOT_ALLOWED with this message:

The 'join_key' field must not start with 'cp_'. That prefix is reserved for pool IDs.

Where can I get more help with ERR_NGROK_SHIP_POOL_JOIN_KEY_NOT_ALLOWED?

Email support@ngrok.com and include the error code.