VPN -> WireGuard -> Local -> Edit. Then copy the public key and send it over to your client. Fill in the information about the server address, port, and specify 0.0.0.0/0 as “Allowed IPs”. On the OPNSense side, choose your client (peer) or multiple peers if there is a need and click “Save”, on the client side tap “Save” icon.
NOTE! Allowed IPs section adds routing tables to your client, so in case of a split-tunnel type of setup, specify all of the allowed internal CIDRs separated by comma.