I would like to connect hestiacp to boxbilling to automate things
I don’t use any billing software so I can’t help you to integrate them to HestiaCP but, you should consider changing BoxBilling to FOSSBilling.
Once installed FOSSBilling you should follow the doc to configure it for HestiaCP.
Boxbilling has not been maintained for over a few years…
So I strongly advise to go with with Fossbilling…