You have to be an admin on a server or use cheats.

Hi Hydromancerx I am currently hosting a multiplayer server. Could you kindly guide me through as to what I need to do to have trade posts?

Press t to open the chat and type in the console command everything within parenthesis excluding the parenthesis ( /additem "Hydrocraft.HCTradingcone" )
You can find a list of server commands here https://wiki.nitrado.net/en/Commands_in_Project_Zomboid
also you can find the item code names listed on the undeniable wiki for hydrocraft and other mods http://undeniable.info/pz/wiki/itemlist.php?wherein=0&pn=1&cat=0&type=0&SEARCH=0
Just remember to specify the mod the item is from in this case Hydrocraft if it is a base item it would be /additem "Base.Axe" etc you can also add a player name if you would like it to spawn in a specific players inventory by default if written like I have with no player name specified it goes into your main inventory.

All previous suggestions are for MP
For SP Necroforge is your friend.
If you don't like temptation of cheating (Necroforge makes it super easy), then after spawning trading cone and placing trading post where you want to, deactivate Necroforge.
That's what I did.

I have a problem with trading post, i have it in my inventory but i cant drop it. When i choose drop command nothing happens.