ERR_NGROK_355: Invalid '_' in domain name - ngrok documentation

What does ERR_NGROK_355 mean?

ngrok returns ERR_NGROK_355 with this message:

Invalid '_' in domain name. Valid domains may not contain underscores. Failed to create an endpoint with the domain <DOMAIN>.

Where can I get more help with ERR_NGROK_355?

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