Don't want to write this by hand? GodCraft sets minecraft:item_hopper for you with a switch.Get GodCraftOverview
minecraft:item_hopper is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Makes the entity work like a hopper, sucking up items into its inventory. Used by the hopper minecart.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:item_hopper": {}
}