If you want a URL that does not change every time you create a new tunnel, you just need to upgrade to Pinggy Pro..
Once you sign in, you will be presented a dashboard where you will find your access token.
Example:
You will also find your persistent subdomain in the dashboard.

Command to start a tunnel to port 8000 and with token xGBTh6cy58q:
Replace 8000 with the port where your service is running.
Replace xGBTh6cy58q with your own token from the dashboard.