No Email Pop Up in Transaction Form

When using a custom transaction form and user noticed that there is no email pop up under Communications tab > Messages, follow steps below:
 
1. Navigate to Setup > Company > Email Preferences > Transactions: USE POPUP FOR MAIN TRANSACTION EMAIL BUTTON = True.
2. Navigate to Customization > Forms > Transaction Form, click Edit on the default Sales Order or Invoice form.
3. On the Tabs tab > mark Communication to Show.
4. Click Save.