If the office proxy only allows outgoing connections to ports 80 and 443 and that can't be changed, you need a forwarder in the internet (outside the office intranet, behind its proxy).
Simplest would be to change the server to listen on port 443 instead or on both 843 and 443. The latter could be achieved by adding a port forwarding rule on the server itself.
If you can't change the proxy or the server, a forwarding rule on any internet server you have access to would do the job. That would be no security problem since it just handles encrypted data. The only drawback is traffic on the forwarding server and maybe its limiting bandwidth.