From the Main Menu, Click on Settings -> Settings -> System Settings.
From the System Settings Menu, Click on 3rd Party Integration.
Click on Authorize.Net.
Check Enable Authorize.Net Integration.
API Key: Your Authorize.Net API Key.
Transaction Key: Your Authorize.Net Transaction Key.
Where do I find my Authorize.Net API Key and Transaction Key?
You can find your Authorize.Net API Key and Transaction Key by logging in to your Authorize.Net account.
If you are unable to find this information, please contact Authorize.Net.