You can access your subscribe form via HTTPS. To do so, change the URL to your subscribe form so that it begins with "https:" instead of "http:".
If you use a custom "pending subscribers" URL
If you have configured your subscribe form (in the Options tab) to redirect pending subscribers to a custom URL, please make sure that the URL you are using begins with "https:". Some web browsers will not allow a subscribe form served via HTTPS to redirect to a non-HTTPS page.