app billing and billing conformation page

[Xiaohua]

Hi, I have build the shopify app. And I would like to add app billing api function into my app. I have used Shopify CLI to generate starter code to build my app. And I check the starter code on folder server/handler/mutations/get-one-time-url.js. there is graphQL mutation code to create charge. But why I open my store, there is no billing conformation page.  I have seen this document https://shopify.dev/apps/billing#best-practices-for-app-billing , but this one only mentioned how to add graphQL mutation create charge code. I would like to know how to add app billing function into app and open the billing conformation page?