You'll need to run the following command first:
sudo netstat -apn | grep tcp | grep LISTEN
Sample output is shown below.
pi@raspberrypi ~ $ sudo netstat -apn | grep tcp | grep LISTEN
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN 2213/nginx
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 2158/sshd
The far right column shows you the application name. In this case there are two active tcp listeners, nginx (a web server) and sshd (the ssh server). You should see your application in this list. Then scan left, and find the section that looks like this:
That indicates a listener on tcp port 80, which is nginx which we found earlier. Whatever number is shown to the right of
is the port number to use for that application.
When you make your connection through the remot3.it portal, you will see a dialog that looks like this:
In this example, the hostname is proxy19.weaved.com and the port number is 34700. Copy those to your VNC client application.