Step-by-step instructions:
For existing customers:
First on the left side of the Truss dashboard, click 'Customers'.
Select the user from the customer list.
Under details, click 'Edit'.
Enter the customer's secondary email and click 'Save changes'.
The secondary email will then be added.
For new customers:
First on the left side of the Truss dashboard, click 'Customers'.
Click '+ New customer'.
Enter the customer's name, primary, secondary email, and click 'Add'.
To note: You can change these details anytime through the customer settings under details.