In EHX, configure an empty port as a Direct (you’ll probably want to avoid the last four ports as we might need VOX gating later), give it a decent talk label.
Go to Control Manager, create a new control (I name them * + radio name, but whatever works) and attach (click from left to right) Frame Relay 1 (or whichever relay you used).
Go back to Matrix Hardware and select the new Direct port. Under Secondary Action, use the drop-down to select the control you just made.
This will mean that any time the port key is pressed, the relay will trigger to tell the handset to transmit.
Go to Panel Programming and add the key; you’ll want a listen key to monitor and a talk key to transmit.