The stack entry provider type adds exactly one item stack to the item group. It creates an item stack with the specified item and an optional set of data components. Item stacks with differing data components are not considered the same, despite having the same item. This means that you can add multiple item stacks of the same items to an item group. This entry provider can also be inlined by only specifying the item.

"minecraft:stick"

{
  "type": "stack",
  "item": "minecraft:firework_rocket",
  "components": {
    "minecraft:fireworks": {
      "flight_duration": 1
    }
  }
}