To add a mod of the steam workshop on your server you must get the numerical ID of the mod.
You usually find this ID in the URL (web address) of the mod in the form https://steamcommunity.com/sharedfiles/filedetails/?id=1359475680&searchtext=
the mod ID in this case is 1359475680

Now edit the autoexec file of your server and add the following line:
LOADMOD 1359475680;

In general you find the line to add in the description of the mod on the page of the steam workshop so you only have to copy / paste.
Do the same for each mod (one line per mod)