The specify calculation option can be used for that:
"Your reference is " YourTable::Field & "contact us if you have any problems"
You can also set up a calculation where you combine text in a text field with data from other fields/tables in your system.
The "template" form letter method in this demo file could be used to produce the body of an email: https://www.dropbox.com/s/6xw8buafjuohncu/MessageTemplateBuilder.fp7
This is an older file, so if You are using FIleMaker 12, open it from the File menu to get a converted copy you can examine.