Stop the server
Open the server in Pterodactyl and select Stop. Wait until it is fully stopped before changing files.
Get a compatible plugin
Download the plugin’s .jar file from its official project page or a trusted repository. Check that it supports your Minecraft version and server software. Avoid unknown re-upload sites.
Upload it to the plugins folder
Open Files in the panel. Find the server’s plugins folder and upload the .jar there. If the folder is missing, confirm the server has started successfully on Paper at least once.
Start and verify
Start the server again. Check the console for a successful plugin load or an error. Many plugins create a folder inside plugins for their settings; follow the plugin author’s instructions to configure it.