Cross-platform local port forwarding solution

Question

I am building a SaaS service which needs to query some data from its user's APIs. Typically user enters a url like mydomain.com/endpoint and service makes requests. However, for some users the endpoint is in their local network behind NAT (say mydomain.local/endpoint)

My idea is to give users the easiest, cross-platform and open-source solution to forward a local port (like 80 or 443) to my server. Done that, I can make queries from server to localhost:12345/endpoint

I can imagine a number of solutions like setting up a vpn (using openvpn) or employing ssh port forwarding (by sshing from local to server with ssh -f -N -R 12345:mydomain.local:80 server.com). However the question is which solution will be the most easy to use for non-professional users.

Ideally, I see it as a cross-platform app (Windows, Max OS, and Linux versions), which is pre-configured by me to talk to my server, get a local url and a desired server-side port (like mydomain.local and 12345 in examples above) and start forwarding.

Update to clarify my question: I'm asking for advice and suggestions which solutions/technologies should I consider in attempt to solve my problem.