Paddle

Log in to your Paddle dashboard and open the developer settings for API keys and notifications.
Create a new private API key.
When generating the key, grant these permissions: Transactions (Read) and Transactions (Write).
Copy the API key into Settings > Payments > Processors > Paddle > API key.
Create a Paddle notification destination using the webhook URL from the admin panel.
Subscribe that destination to transaction.completed, then copy the generated endpoint secret key into Settings > Payments > Processors > Paddle > Webhook secret.
Go to Checkout > Hosted Checkouts in Paddle and create or open the hosted checkout used for this integration.
Set the hosted checkout return URL to the URL shown in the admin panel. The webhook above should stay enabled because it is used for the final payment confirmation.
Copy the full hosted checkout link, for example https://pay.paddle.io/checkout/hsc_..., and paste it into Settings > Payments > Processors > Paddle > Hosted checkout URL.