Firstly you need to add the @folder provided to the root of your server.
Now that you have done the above you may also need to add keys to your key folder.
Once all the above is done all you need to do is find the server manager.
You can find the server manager on your control panel it looks like the image below:
Now you have found the server manager you need to select advanced in the Access Level.
This will then give you more options like the image below:
Now you will need to add the @folder to the client mods section like the image below:
Note) If you have multiple server addons you will need to separate them with ;
With all the information above you should now be able to add Client Side Addons to your server.