Sendy integration for WP Contact Form 7 (https://wordpress.org/plugins/contact-form-7/) that we use for Awesome Presentation (awesome-presentation.com). Because the other plugins were featureless, complicated and mostly people use the Contact Form plugin anyway.
It's pretty easy and automatic. Just change the line to your Sendy location:
$sendyUrl = "http://www.yoursendy.com/subscribe";
Then use the Contact Form 7 to build you form like so:
<p>Name *<br />
[text* name] </p>
<p>Your Email *<br />
[email* email] </p>
<p>[acceptance acceptance-175 default:on] Yes! Sign me up for the BETA list!</p>
[sendywpcf id=""]
[quiz quiz-44 "4+2=?|6" "2+10=?|12" "X+Y=?|Z" "33+33=?|66" "A+B=?|C"]
<p>[submit "Sign ME Up!"]</p>
In the shortcode [sendywpcd id=""] add in your list id from Sendy. I recommend a quiz, and acceptance checkbox. I also recommend double opt-in on the Sendy side :)