ERR_NGROK_403: The first subdomain of a wildcard domain must be a single '*' character - ngrok documentation

What does ERR_NGROK_403 mean?

ngrok returns ERR_NGROK_403 with this message:

The first subdomain of a wildcard domain must be a single '*' character. Failed to reserve <DOMAIN>.

Where can I get more help with ERR_NGROK_403?

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