Aweber Double Opt-in

Aweber automatically uses double opt-in on 99% of API integrations as default irrespective of your list settings. You can contact customer services at Aweber to turn it off for you.

Check out this article for more details: https://help.aweber.com/hc/en-us/articles/360002536994-Can-I-disable-confirmed-opt-in-for-my-integrations-

We now ONLY support Aweber via API since the form code used for Custom Code includes a Captcha which cannot be presented to the viewer by the program - so the subscribe will fail.