Don't want to write this by hand? GodCraft sets minecraft:balloonable for you with a switch.Get GodCraftOverview
minecraft:balloonable is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Lets a balloon be tied to this entity so it can be lifted into the air.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:balloonable": {}
}