How create payment session by graphQL or REST in development shop for testing payment mutation?

[BlenderPay]

• We are developing a new offsite payment app.
• We registered this app on the Shopify Partner page and received a Client ID and Client secret.
• We created an extension for test_pay_app and provided the Payment session URL and Refund session URL (on our development server).
• We added the URL to the Allowed redirection URL(s) for correct OAuth processing.
• We created a test shop  and installed our test_pay_app app and extension on this shop.
• We verified the authentication method and received the correct access token for executing GraphQL queries.
  
  However, we do not know how to test the payment session mutation.
  In the general case, the first request to initiate a session is sent to us by Shopify (Offsite payment flow begins with an HTTP POST request from Shopify to the payment session URL of our offsite payments app extension provided during app extension configuration.
  
  This request contains information about the customer and the order, according to the documentation). In this request, we receive the session ID.
  
  But how can we emulate this process before we submit our payment application for approval?
  
  We thought we could choose our test payment app as a payment method on our test shop, simulate orders and payments through the test shop site, receive the first request from Shopify, obtain the payment session ID, and check how our code executes the payment session mutation. However, this is not possible.
  
  We received the following message in our extension:
  “Your payments app can’t be installed on a store. You need to create a new app version and submit it for review. Once it’s approved, you can release the app version to start testing it on a development store.”
  
  In this case, do we need to submit an untested app for approval? Please explain how this process works. How can we develop and test the app before requesting approval? What do we need to do? Thanks.