How do I get the block from item use on block advancement

I have this code:
{
  "criteria": {
    "requirement": {
      "trigger": "minecraft:item_used_on_block",
      "conditions": {
        "player": [
          {
            "condition": "minecraft:match_tool",
            "predicate": {
              "items": [
                "minecraft:player_head"
              ],
              "nbt": "{custom_data:{seed:1b}}"
            }
          },
          {
            "condition": "minecraft:location_check",
            "offsetY": -1,
            "predicate": {
              "block": {
                "blocks": [
                  "minecraft:farmland"
                ]
              }
            }
          }
        ]
      }
    }
  }
}

That is my advancement that checks if the matched tool is a player head with tag seed which is also ontop of farmland.
How would I go about replacing that matched tool to a different block?