You cannot ride a ghast in vanilla Minecraft
Use commands or mods if you want to make it possible
Enable cheats in your world
Spawn a ghast with a command
Use a mod, datapack, or plugin that allows riding mobs
If using commands, try summoning a rideable custom entity setup
Mount the ghast using the mod or custom command system
Control it with the mod’s riding controls or custom behavior
In survival without mods or commands, it is not possible