To add an account to your subscription (you may add up to a maximum of 5) follow these steps:

1. Log in to My API Console - http://console.bdp.betfair.com/

2. Click on My API Subscriptions, then your subscription 

3. Click on 'Click here to manage your API Sub-Accounts'

4. Click 'Add Sub-Account'

5. Insert the username of the account you wish to permission, then click 'Add'

6. Your account is now permissioned and you can log into the API with productId 0 and vendorSoftwareId as displayed in My API Console.