To set up a TLS passthru tunnel to port 8443, request a port forward from port 443 by running the following command:
ssh -R 443:localhost:8443 localhost.run
localhost.run will forward all connections to your domain on port 443 thru to your app as is, and your app will need to provide it's own certificate and decrypt the connection.
If you don't want to handle certificates and decryption yourself but still want your clients to connect to HTTPS see HTTP Tunnels, they provide HTTPS endpoints and take care of decryption for you.
This mode of operation is currently only available to Custom Domains.