Don't want to write this by hand? GodCraft sets minecraft:is_illager_captain for you with a switch.Get GodCraftOverview
minecraft:is_illager_captain is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Marks the entity as an illager captain, the raid leader that carries a banner.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:is_illager_captain": {}
}