CS:GO servers require a login token to connect to the masterserver. To create a token please follow those steps:

1) Navigate to the following page and login -> https://steamcommunity.com/dev/managegameservers
2) Enter 730 into the App ID field.
3) Enter a note in memo field if you want to. (for example, My Deathmatch Server)
4) Click on create
5) Copy the login token into the field in the webinterface

For an existing Basic server:
Perform step 1-4
5) Edit configuration -> Edit Game Configuration -> Server Settings -> Gameserver Login Token

Insert the logintoken and save. Restart the server.