It's good to see again here in the Community, J_S_R.
Allow me to take over and provide some insights regarding inserting fields in QuickBooks Desktop email template.
Adding your own statements on the email template is currently unavailable in QuickBooks Desktop. For now, you can check the screenshot below for the available fields to use for the template.
To know more about customizing your email template in QuickBooks, check out this article: Create custom email templates in QuickBooks Desktop.
The Community is around your corner if you have any other letter template concerns. Have a great day.