Item Modifier Context

[
  {
    "function": "minecraft:copy_custom_data",
    "source": "block_entity",
    "ops": [
      {
        "source": "Items",
        "target": "ulg.BackPack.Inventory",
        "op": "replace"
      }
    ]
  }
]
Hi! I'm updating my datapack to 1.21 changing some of the code logic.
In this case, setting the source as "block_entity" (I'm using misode generator), I can only specify the source path of the value and not the block entity (a barrel) position.
How should I do that (specify the block entity pos)?