Don't want to write this by hand? GodCraft sets minecraft:attack_damage for you with a switch.Get GodCraftOverview
minecraft:attack_damage is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Sets the base melee damage of the entity. Cats and ocelots use it for their scratch.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:attack_damage": {}
}