It’s done through the GUI, see Authentication in the screenshot in FreePBX + Didww pjsip outbound trunk registration fails (note the rest of the thread won’t relate to your specific case - I would have used the user guide, if I could have found it).
This will only work if the provider is being correctly recognized, and the setting for that is on the Advanced Tab (Match / Permit), if they send from a different address from that which you use to send to them.