You can add additional fields to the transaction by clicking Add field in the Transaction fields under Fields tab.
Once you click Add field button a pop-up window will occur where you can set the field type and check the box if you want the field to be required. You can make the field to be mandatory optionally depending on the status of transaction.
This fields will show up under Additional fields anytime a user creates a new transaction. If you set the field to be mandatory it will be marked red and agent won't be able to proceed without filling this information.