Unable to set external_signaling_port in freepbx

I’m new to freepbx.
I’m trying to set a NAT configuration for pjsip and chan_sip using non standard port.

The only way to be able to make it work was to explicit a port for externip (like: externip= and external_signaling_port=5321.

Unfortunately I’m unable to find any way to set external_signaling_port in freepbx. If I add it to pjsip.transports.conf (under []) manually it is deleted every time I click “apply” in the GUI.

Any idea on how to do it?