How to have a mob drop occur only when a TNT kills them.

I want a custom drop to happen when a creeper is killed by a TNT. I've tested a couple of methods already and none have seemed to work this is the current code I'm working on (note this is in a list of conditions): 
"conditions" : [
            {
              "condition": "minecraft:damage_source_properties",
              "predicate" :{
                "direct_entity": {
                  "types" : [
                    {
                      "type" : "minecraft:tnt"
                    },
                    {
                      "type" : "minecraft:tnt_minecart"
                    }
                  ]
                },
                "source_entity" : {
                  "types" : [
                    {
                      "type" : "minecraft:tnt"
                    },
                    {
                      "type" : "minecraft:tnt_minecart"
                    }
                  ]
                }
              }
            },
            {
              "condition" : "minecraft:random_chance",
              "chance" : 1
            }