This VAS is described in section Sending fax without fax machine.
Al far as this VAS is used from User web-interface, it's assumed, that user has been already added to the system.
To add a user, you can use instructions from section Configuring PBX.
This VAS is enabled by default, in case if PBX is configured according to instructions Configuring PBX.
Also, need to make sure web-access is granted for users which need this feature.
Step-by-step instruction on providing web-access is located at section Providing web-access to user.