minecraft-assets/data/minecraft/recipes/lead.json

{
  "type": "minecraft:crafting_shaped",
  "category": "equipment",
  "key": {
    "O": {
      "item": "minecraft:slime_ball"
    },
    "~": {
      "item": "minecraft:string"
    }
  },
  "pattern": [
    "~~ ",
    "~O ",
    "  ~"
  ],
  "result": {
    "count": 2,
    "item": "minecraft:lead"
  }
}