What is the data value for a thick splash potion?

I had a look at the vanilla behaviour pack, which can be downloaded from the Minecraft Add-Ons page.

I found this recipe for creating a potion of weakness using a thick potion in recipes/brew_thick_fermented_spider_eye.json:

{
  "format_version": "1.12",
  "minecraft:recipe_brewing_mix": {
    "description": {
      "identifier": "minecraft:brew_thick_fermented_spider_eye"
    },

    "tags": [ "brewing_stand" ],

    "input": "minecraft:potion_type:thick",
    "reagent": "minecraft:fermented_spider_eye",
    "output": "minecraft:potion_type:weakness"
  }

}

It seems like you should use "minecraft:potion_type:thick" for the input for your brewing recipe.