You will have to port forward your Pi on your router. To do this, follow these steps:
On a laptop or PC running Windows, open Command Prompt and type 'ipconfig'. Look for your Ethernet or WiFi adapter. Note down the Gateway address. Next, go into your browser and enter that address. If prompted for a user name/password, look on the bottom/side/back of router for this. Once logged in, look for "Applications & Gaming" or similar. Find the "Port Forwarding" tab. You may need to look up specific instructions for your router. Then, add the IP address of the Pi in the "Local IP" box. For internal ports, put what you use to connect to the motion stream. For external, you can pick whatever. If there is an option for TCP/UDP, select TCP. Enable it if there is an option for "Enable". Save changes.
Now, you need your WAN IP. Simply go to "whatismyip.org" and note down that address.
Finally, type the WAN IP and your Port (separated by a : colon) and hit enter. The stream should show up, but if your upload speed is slow the stream may lag. Also check with your Internet Service Provider (ISP) to see if hosting a stream is within your Terms Of Service.
Hope this helps :)