After configuring the Metafields External/ Affiliate in Settings > Metafields, next step, you need to go to Products > Open the Product that you wanna add the Metafields External/ Affiliate button. Kindly follow my GIF image below to know how to add the Metafield External/ Affiliate for the product.
And when you view the product, you will see the External/ Affiliate button show like the image below: