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.

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