Loot Pool Custom Data

I have a ui item determined by a loot pool. I want it to have minecraft:custom_data={"tag":"trade_station_ui"}. However every which way I try it, it doesn't work.
I've tried
    {
      "function": "minecraft:set_custom_data",
      "tag": "\"tag\":\"trade_station_ui\""
    }
  ]```
and
```"functions": [
    {
      "function": "minecraft:set_custom_data",
      "tag": "{\"tag\":\"trade_station_ui\"}"
    }
  ]```
and even
```"functions": [
    {
      "function": "minecraft:set_custom_data",
      "tag": "'{\"tag\":\"trade_station_ui\"}'"
    }
  ]```

The loot pool works fine until I add this (straight from the generator I might add), What am I doing wrong?