ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_TRACKING_REF_REQUIRED: The 'tracking_ref' field must not be empty - ngrok documentation

What does ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_TRACKING_REF_REQUIRED mean?

ngrok returns ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_TRACKING_REF_REQUIRED with this message:

The 'tracking_ref' field must not be empty.

Where can I get more help with ERR_NGROK_SHIP_CONTINUOUS_DELIVERY_TRACKING_REF_REQUIRED?

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