To install plugins on your Minecraft server, open the More menu in your server panel and go to the Plugins page. There, you can browse available plugins, select the one you want, and install it directly from the panel.
This page is designed to make plugin installation much easier, so in most cases you do not need to download plugin files manually or upload them through the file manager.
After installing a plugin, restart your server if needed so the changes can take effect.
If you need help with this, please submit a ticket.