To install mods:
1) Stop your server in the Game Panel.
2) Click on the Configuration Files tool.
3) Click Text Editor for hosting.cfg
4) Look for 'steam_ugc=' and write the mod ID's. You should space them with a comma. For example, three mods would look like 1111111111,2222222222,3333333333
5) Click save at the top
6) Start the server
You can find the Mod ID of the mod, opening the workshop page of the mod, and copying the numbers in the end of the workshop page link, example: https://steamcommunity.com/sharedfiles/filedetails/?id=1673502088 the ModID it's 1673502088