Don't want to write this by hand? GodCraft sets minecraft:pushable_by_block for you with a switch.Get GodCraftOverview
minecraft:pushable_by_block is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Lets moving blocks such as pistons push this entity.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:pushable_by_block": {}
}