Install Hytale mods and plugins

Back up before adding anything new, see backups. Version mismatches are the top cause of crashes, so every file must match your server’s game version.

On the server

Download the mod or plugin, usually a .jar, from a trusted source such as CurseForge.
Stop the server in the panel .
In Files, upload gameplay mods to the mods folder and server management plugins to earlyplugins (create the folder if it is missing).
Start the server and check the startup log for a message that it loaded.

On your PC

CurseForge app: choose Hytale, search for the mod, press Install, then Play.
Manually: put the .jar in %appdata%\Hytale\UserData\Mods, creating the Mods folder if needed.

If the server crashes on startup, remove the newest mod and check it matches the game version. For a bug in one plugin, contact its author.