While I wouldn’t try it myself, I’d imagine you could do this by forwarding 443 out to your ddns, then accessing it from the app by putting in the domain you’ve set up for it (just as you would with your others, server.domain.com) instead of your local IP.
My own .02 though, feels like a bad idea, exposing your entire server, the full kernel I mean, to the outside world without a VPN, API key or no.