You don't.
When you release on Steam, they will make you have your game cost that minimum price everywhere. It is not limited to giving away Steam keys, as far as I know.
But since you already seem to have a Steam page, you should have all the documents available to check in detail what you can do and what not. Maybe I am mistaken.
Also, the individually priced files mechanic is not a good tool. It is prone to misunderstandings. It might qualify for having a minimum price according to Steam tos, if you only make available the full game files for that price. But better double check.
The external key mechanic is for "owners" of your project. https://itch.io/docs/creators/download-keys#external-keys-steam-etc
You become an "owner" no matter how much you pay, as long as you pay any money. Even if it is below the price of any individually priced files. That means, any of your pay what you want customers will count as owners, when you set a minimum price later. You can think of them as having bought the game on a discount.