My web server is running on localhost (127.0.0.1:8800) and I would like to send HTTP posts from another machine to that server.
Does anyone know how I will be able to access the server from the outside? (I did a SSL port forwarding from the external address to localhost, but it is a kind of hack and some aspects do not working properly, e.g. web socket does not detect traffic that's been forwarded)
127.0.0.1 is the loopback address of your network card. Only your computer can access that. 127.0.0.1 on any other computer will be that other computer.
You will have to run your server on your internal ip-address (if you're doing this in an internal network). It will probably look something like 192.168.0.x. If you're instead connecting to the computer using the internet you will of course have to bind the server to the external ip-address.
You can find your internal ip-address on Windows by opening cmd.exe and typing ipconfig. On OSX or Linux/BSD you run ifconfig in a terminal.