You can simply include what should be configurable by your users in the “settings” under “schema” of your app block.
The settings will be saved and managed by Shopify and you can read is using liquid syntax.
See this example in Shopify’s documentation: https://shopify.dev/docs/apps/build/online-store/theme-app-extensions/configuration#example-app-block
Alternatively, you can manage the values in your own app’s backend where users will set the value in your app and you can do an API fetch call to your own app to fetch the value.