This is a fantastic question and one that highlights our need to improve our documentation. The instructions for setting this up are in the Configuration docs, but I think they should be elevated to an actual step in the installation process. So thank you for pointing this out.
Whenever we need to generate a absolute URL (usually for emails) we employ the Grails “createLink” feature that uses the grails.serverURL configuration property.
The default value for this is a URL that points to localhost.
In order to change this for your implementation simply edit openboxes-config.properties