First of all retrieve your device MAC Address from the bottom of the box and send it to your IPTV provider for activation.
After that they should provide you with an address called Portal Address, then do the following:
 Updated on Nov 26th, 2022 
Note: We don't support Stalker Portal anymore. This post is just informatory.

1-Reboot your device by powering it off and on
2-While booting, once the loading image pops up, press set/setup button to load portal
3-To open up servers section, press set/setup button again
4-Go to "Portal URL 1", press the "KB" button and type the portal address you were given by your provider.
5-Press the "KB" button again and then choose OK to go back to the previous page
6-Now go to "Reload Portal" and then press OK
A detailed guide can be found here on How to add an IPTV portal on a MAG device?.