How do I open the TPL Code Editor for a Form Type?
In order to add TPL to events for fields or buttons, you must enable custom code for the form type.
Turning on custom code for a form type enables access to the TPL editor.
On the Designer tab, select the form design you wish to customize.
Expand the Advanced Features section, locate the Enable custom code for this form type property, and select Yes.
Finally, click the Edit Custom Code button to open the TPL Editor.