Authentication API to grant access for other Roblox APIs?

I am working on a plugin and I want to load the player's inventory, the only problem is that at the moment it only works with inventories which are not private. I came across the Roblox authentication API and I am trying to figure out how I can make the script grant access to the player's inventory if it belongs to him and it is private.

So far I have tried to get the Auth meta data but I am not sure how to use it for my needs and when trying to use it to login (as I obviously can't get the player's credentials) it prints HTTP 403 (HTTP 403 (Token Validation Failed))

Can anyone guide me into the process of making the server grant the player authorization to APIs which need you to be logged in to access?