Don't want to write this by hand? GodCraft sets minecraft:behavior.enderman_take_block for you with a switch.Get GodCraftOverview
minecraft:behavior.enderman_take_block is an entity component: it goes in the components object of a custom mob, or in one of its component groups. AI goal that lets an enderman pick up a block and carry it around.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:behavior.enderman_take_block": {}
}