When sending out a template to a customer you can add properties in the variables section while inserting the inputs in the following pattern: {contact.PROPERTY_INTERNAL_NAME}
The internal name can be found in Settings→Contact Properties, under Internalname.
This can also be used while sending out broadcast messages, In the following format:
Want to print your doc? This is not the way.
Try clicking the ⋯ next to your doc name or using a keyboard shortcut (