To get started, log into the dashboard and navigate to the Apps tab on the sidebar. Then, click on the existing app that you'd like to add payments to as part of the App scope or you can search for "Apps" in the search bar.
Next, click on the app and a new page will be displayed after you do. At the top right of the page, you will see the "Edit" button. Click on the button to bring up the edit modal.
Next, under "Account Scope", select "Payments" and click the "save changes" button.
Once this is done, DirectPay will be added to your account scope.
Finally, you can then use your App secret key which you pass in the DirectPay API header to launch DirectPay in test mode.
You can also check out this article on how to test DirectPay in sandbox mode.