There are two ways to add a controller for a customer.
The first method is to create the controller on your own account. Then, configure it as you would if you or the customer created it straight on their account. This option can be used when a customer does not give account access to the contractor.
As a contractor, you would create additional controllers just like you would any other time.
Go to Settings > My Account.
Then go to My Controller Summary and click Add Controller.
Create the Controller Name and enter the serial number (if you have it). Otherwise, this can wait for when you get the customer's serial number.
Next, select the account in which you want to create the controller. In this case, because you do not have access to the customer's account, you will need to create the controller in your own account. If you end up gaining access to the customer's account, you can create it straight in their account. This will save you from having to move the controller from your account to theirs at a later time.
To learn more about moving controllers from your account into your customer's account, click here.
The second way to add a controller to a customer's account would be to create it directly by selecting the account under Customer Account.