For the record and in case anyone is searching the forums, let me help:
Go to Advanced Call Flows, then Fax Boxes. From there, identify your Main Fax box and select the Advanced tab. Under the "SMTP" tab, you can use either the "SMTP email address" as the messaging domain or if your administrator has configured a custom or white label address (as I have for you), you can use the messaging domain. Let's say for the sake of discussion that this domain is 23423.fax.zswitch.net.
To gateway your fax via email, create a message to your destination fax number, let's say it's 520-123-4567 and create the to address: "firstname.lastname@example.org" and attach your PDF fax to the message and send.
This same process should work for any Fax Box on the system. Don't forget to review the "SMTP permission list" to see who is allowed to use the gateway.