Please navigate to your Merchant dashboard, find the section called “API keys” and click "Activate" .
In order to activate your API key and see your secret enter 2FA code and click "Activate".
After the activation you will see your secret.
Please note that after closing the "Activate API key" window, you won't be able to see it again. Your secret will be hidden and you won’t be able to restore it, please save it in a secure place and avoid sending it via messengers or even email.