‘Go to’ the Merchant Account under the ‘Payment Gateway’ section (If you are first-time-login then you will see the ‘Become a Merchant’ option.)
After the merchant account login, click on your profile picture.
Select ‘Test mode’.
Fill user credentials like username and password for your sandbox account and click on the sandbox login.
If you have already active sandbox account and you have forgotten the password, then you can change your password by clicking on update password button.