Create new Steam API key

You need a valid Steam account

http://steamcommunity.com/dev/apikey

Create a new file in your CSGO server directory

webapi_authkey.txt

C:\csgoserver\csgo

Type in your Steam Web API key

Now start your server

You can change to a workshop map by using this command

host_workshop_map 644183353

You can get the steam workshop map ID via your browser