Don't want to write this by hand? GodCraft sets minecraft:balloon for you with a switch.Get GodCraftOverview
minecraft:balloon is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Makes the entity act as a balloon that can be tied to other entities and float upward. Used by the Education Edition balloon.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:balloon": {}
}