|When creating a form, how can I add a hidden field that will show up on the email notification that the list owner gets when a new contact joins an email list?|
Create a new custom field (eg "form referrer") as a text-field.
Include it in your form.
When you "Get HTML" you can change it to a hidden field.
For example change:
<input type="text" name="CustomFields[XX]" value="">
<input type="hidden" name="CustomFields[XX]" value="http://www.domain.com">
and it will be processed as normal with the other custom fields.
Make sure you only change the "type" and not the name of the field itself.