Don't want to write this by hand? GodCraft sets minecraft:bucketable for you with a switch.Get GodCraftOverview
minecraft:bucketable is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Lets players scoop the entity up with a bucket, like fish and axolotls.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:bucketable": {}
}