Don't want to write this by hand? GodCraft sets minecraft:absorption for you with a switch.Get GodCraftOverview
minecraft:absorption is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Gives the entity extra absorption health: bonus hearts that soak up damage before its normal health goes down.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:absorption": {}
}