To load a map from the steam workshop on your server you will first have to go on the workshop page of the map.

In the URL (address of the page) you will find the code of the map which is located at the very end of the address.

For example, if we take the following address:
Https://steamcommunity.com/sharedfiles/filedetails/?id=821096827

The code of the map is 821096827

Now that you have the map code go to the your game panel > config files and edit the autoexec.cfg file
You will find a line starting with host followed by the port number of your server, for example:
Host 12871
The port number may be different on your server, in any case never change it!

Simply add on the same line the code of the map preceded by w-

So for our example map:
Host 12871 w-821096827

Save the edited file and restart the server. The map will be automatically downloaded from the steam workshop and loaded by your server when it starts.