To use workshop maps with your CSGO server, you will need 2 pieces of information.
To grab these pieces of information, they can be obtained from the URL of the steam workshop page.
Workshop Collection: http://steamcommunity.com/workshop/filedetails/?id=abcd1234
Starting Map ID: http://steamcommunity.com/sharedfiles/filedetails/?id=abcd1234
The text highlighted in red are the IDs you will be using for your configuration.
Now that you have your 2 IDs, log onto our game panel and select the server you wish to configure.
In the "Settings" page, you should see 2 fields.
Fill in the relevant information and restart your server.
NOTE: please remember to set the Workshop Collection if setting the Start Map or your server will not run