Loudspeaker paging interfaces via a Trunk port, and the paging button is usually Pickup and the paging trunk number.

So if the trunk is 804, the button would be Pickup 804. At the phone, SPKR, #00, Button, *9-804, SPKR

I have seen programmers put the paging trunk itself on a button, which also gives you the status via the light. Unless the button is located directly above the SA buttons, it's trick to place on the telephone set.

ps, I usually renumber the paging trunk from 8xx to 63. Then loudspeaker paging would be Pickup Line 63, which can be dialed on the keypad as #963, straight up the right hand side of the dial.