When in a message template, the form drop-down on the right-hand side and shortcode (i.e. [form /]) only works with fundraiser-type data capture forms. That means you will only see fundraiser-type forms appear in this drop-down and not event registration/event registration with 1-click type forms).
To add an event registration form to a message template, do the following:
Go to Campaigns in the main navigation bar at the top of your account and find the appropriate Campaign
On the right-hand side is a column titled ''Registration Form''. It is in this column where your event registration form URL will be located. Copy this to your clipboard.
Go to Messaging in the main navigation bar at the top of your account and select templates from the drop-down.
In your message template paste the URL of the form where you desire it.