This web app has multiple functionalities:
To get started, you need the REWARDTABLE.EXML file. It can either be found in METADATA/REALITY/TABLES of your unpacked game files, or alternatively at the same path in the NMS EXML Archive.
To find out what item a specific product ID corresponds to, or to find the product ID of a specific item, you can use the Assistant for NMS web app. On every item is a "<>" icon in the top left of the in-game icon. When clicked, it displays the "GameId".
You don't need to upload the REWARDTABLE.EXML if you are using the "Paste EXML" input.