Shared Mailboxes on Asterisk 11.17.1 - FreePBX 12.0.68

Continuing the discussion from Set up Shared Mailboxes:

To create a feature code prefix, no editing of conf files is required, you can do it in the GUI.

Create a Custom Destination to dial digits minus the first two and prepend a :

Create a Misc Application that dials underscore, 9, 7, period:
and point the Misc Application to the Custom Destination.