There is documentation on writing your own custom submit. Google on Custom Submit adobe Adaptive forms. If that does not help let us know, will send you sample package
Thank you very much for the tip.
1. but I only want to edit/add-in the built-in email submit action. Would you happen to know where I can find the code for that? somewhere in /libs?
2. Also a sample submit action in zip file would be most welcome.
3. what I'm trying to do is to implement Google reCaptcha on an adaptive form that send an email. I haven't found any answers to this one yet.
What customization are you planning to do ? I am not sure if you can customize the outbofbthw. If submit actions
the best option is to write your own custom submit action
1. You can find the out of the box email action code under /libs/fd/af/components/guidesubmittype/email
2. You can use overlay/override concept to write custom submit action.(URL: https://helpx.adobe.com/aem-forms/6/custom-submit-action-form.html). For sending email use HtmlEmail class .