In order To create API credentials for WHMCS to automatically detect the receipt of payments, you need to create a webhook in Stripe.
Open the Webhook menu and click on the button "Add endpoint":
You must fill in the input fields as follows:
After clicking "Add endpoint", Stripe will redirect you to the overview page of the webhook. Click the button "Click to reveal" at "Signing secret", you need to insert this value at the WHMCS payment configuration page ("Webhook Secret Signature")., login into your Klarna Account and open the page "Settings" → "Klarna API Credentials".
On the API credentials page click the button "Generate new Klarna API credentials" and insert the username and password at the WHMCS gateway configuration page.