In order 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").