Datapacks are kind of like modpacks but only add small modifications to the game. An example of this would be the HermitCraft server.
1) Visit your FTP File access
2) Open your world file
3) Upload your datapack into the "datapacks" folder (It should be in a .zip format)
4) Reboot your server.
You can see if it worked by typing in game: `/datapacks`