Modpack - Starter Backpack with items

[mistercapitale]

Hello, I'm currently creating a modpack, my idea was to start with the basic backpack and it has some items inside it. Im using ChatGPT to help with the coding. I can get the items to auto populate in my items bar but can't get them to be inside the backpack. Is this possible? If so, how do I get it to work? I appreciate any help anyone can give. Thank you.

[mistercapitale]

I have the code for the backpack: just need to understand how to get an upgrade to show up in the backpack.
// 🎒 Basic Netherite Backpack
player.inventory.add(Item.of('sophisticatedbackpacks:netherite_backpack'));

I want a sophisticatedbackpacks:stack_upgrade_tier_4 to be in upgrade slot.

I would highly appreciate your help on this @P3pp3rF1y Ive searched to try and find info on this anywhere and haven't been lucky.