How do I add Add-ons when purchasing a new Subscription?

When purchasing a new Subscription, if the parent license has suitable Add-ons, in the next step you will be able to choose them.
1. Select the "Add" button and you will open the following table, where you can also change the quantity and choose the charge period: 

2. After Add-ons (if any) are added to parent license, confirm your choice:

3. After confirmation, a pop-up window with a purchase summary will appear on the right side of the screen: 

Follow the next step to complete the purchase. How do I modify my Order?