Regarding your question, which interface are you referring to, please? if it is a self-hosted or a full 3cx deployment and you have management console access, then trunks are configured under the Sip trunks section. https://www.3cx.com/docs/manual/sip-trunks/.
If your system is a Start up version, then this is found under the office settings, when logged in as the manager of the system, Voice & Chat section > +Add Trunk: https://www.3cx.com/docs/startup/#h.k9k10ro9oev9