To create custom items using addonpacks, you have to create a file for them in addon/<namespace>/items/<item ID>.json. After that check the items.html (in mods/documentation/palladium) file for all the settings you can add onto your item, this wiki page will not cover them.

If you load the game up now, you will notice that your item has no model/texture. To provide that you need to create following file:
assets/<namespace>/models/item/<item ID>.json

{
  "parent": "minecraft:item/generated",
  "textures": {
    "layer0": "test:item/example_item"
  }
}

test:item/example_item would be the path to your item texture.

If you want to place your item in a creative mode tab, there are multiple ways.
If the tab is your own, you can directly list your item in the creative mode tab json file, or you set the creative mode tab ID in the item json file like this:

"creative_mode_tab": "example:my_creative_tab"

You can however also place it into Vanilla's tabs at precise positions, like this:

"creative_mode_tab": {
   "tab": "minecraft:food_and_drinks",
   "after": "minecraft:bread"
}

This would place it into the foods-tab AFTER bread. The reverse is also possible:

"creative_mode_tab": {
   "tab": "minecraft:food_and_drinks",
   "before": "minecraft:bread"
}

You can also combine all of the above into an array and place it into multiple tabs:

"creative_mode_tab": [
   "example:my_creative_tab",
   {
      "tab": "minecraft:food_and_drinks",
      "after": "minecraft:bread"
   }
]