You can install plugins only on Spigot and Bukkit servers.
Standard Minecraft, Forge and Modpack servers don't support plugins.

To install plugins you will need to get .jar files.
Search and download Minecraft Server plugins here:
https://www.spigotmc.org/resources/

As an example, we will describe the NametagEdit plugin installation process: https://www.spigotmc.org/resources/nametagedit.3836/.

Go to the Your Gameserver → File manager page, open folder "plugins", click Upload and upload your mod .jar files.

Reboot your game server server to start using the new plugins. By using the Console (Your Game server → Console page), run the "plugins" command to check the correctness of plugins installation. The output will show a list of all plugins installed.