These are the instructions that Authorize.net are providing for obtaining the transaction key:
- Log on to the Merchant Interface at https://account.authorize.net
- Select Settings under Account in the main menu on the left.
- Click API Login ID and Transaction Key in the Security Settings section.
- Enter the secret answer to the secret question you configured when you activated your user account.
- Click Submit.
The Transaction Key for your account is displayed on a confirmation page. You can read more on their online guide here.