You cannot tame a ghast in vanilla Minecraft
Ghasts are hostile mobs and cannot be turned into pets
Use a lead only if a mod or addon allows it
Use commands in Creative mode if you want to spawn or control one
Install a mod or datapack that adds ghast taming if you want a tameable ghast
Trap a ghast in a safe enclosure if you want to keep it nearby
Feed or interact with it only if a mod specifically adds that feature