The form drop down and short-code (i.e. [form /]) in message templates only works with Fundraiser Data capture forms not with event registration forms.
To add an event registration form to a message template do the following:
- Go to Account Settings > Events
- Select the Event and click on the row to edit it
- On the right hand side is the drop down of Event Registration forms, make sure the correct form you want is selected
- Click the pink "Copy" button to copy the URL of the event registration form to your clipboard
- Go to Account Settings > Messages
- In your Message Template paste the URL of the form in the message template where you desire it.