is this a rented dedicated server?
if so, you need access to the "game files" to add mods to the server.
some server providers may provide a way to access those files and add mods.

if this is your OWN dedicated server (like you have a spare computer running as "the server"), you just add the mods you want to use the same way you would normally.

and finally, after setting up all the mods on the server..... ALL PLAYERS WILL NEED TO HAVE ALL THE SAME MODS AS THE SERVER.