Mandatory information about my setup:

• Have I searched the tickets before posting? Yes
  
• Have I searched the forum before posting? Yes
  
• Have I read the documentation before posting (which pages?)? Yes. All of them.
  
• Joomla! version: 3.4.0
  
• PHP version: 5.5.17
  
• MySQL version: 5.5.34-32.0-log
  
• Matukio version: 5.1.3
  
• Direct link to the page that shows the issue: Not relevant
  

Description of my issue:
Please allow me to refer to the forum post Use PHP code directly in the Matukio Templates.

In short, I'd like some guidance on how to add an extra placeholder that calculates a structured communication, based upon the invoice number.