How can I add custom fields to a payment gateway app on Shopify?

I’m in the process of integrating a payment gateway with Shopify.
A public app was created and a payment extension was created and was approved by Shopify. Now I need custom fields to be added
in order for the merchant to fill after installing the app. Which is required for us. How can I achieve this?

Requested fields,

App ID
Hash Salt
Token

@Anonymous