BoltMail
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="">

to

<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.