To do this you will need to do the following:
- Visit here http://steamcommunity.com/dev/managegameservers
- Once logged in, proceed to generate a new token (the app ID for CS:GO is 730).
- Once generated and you've got your token go to configuration files in the control panel.
- Click text editor to the right of your server.cfg file
- Add a new line with sv_setsteamaccount "your-generated-token-here" and then Click save.
- Restart your server and you are now done.