I made an extension and I  want to use merged fields for a template.

For example, I made this module called 'Template Test'. 

I made a field called 'Message Template' like this: Hi  ${Contacts.Last_Name}  ${Contacts.Full_Name} ${Contacts.Phone}  ${Contacts.Owner}.

The problem is these values are not merged. In my function I get ${Contacts.Last_Name} not the actual Contact Name.