...
The Validation & Preview button, when clicked, will also populate an iframe in the Preview tab displaying your rendered html code.
Some rules about the preview tab:
...
On the Email Template tab (first tab), click the 
button.
Two next text fields will appear, one for a name, and the other for a default value. The name is required, the default value is optional. Do not add the asterisk or pipe in the name.
You may continue clicking the button to add additional parameters. There is no limit to the number of parameters you may add, but common sense should rule the day. An email with fifty parameters is probably too complex. You must click the Save button when finished to make the parameters permanent.
Opt-Out Parameter
...