This plugin allows you to easily embed Web forms built with Formstack's online form builder into your sidebar, pages, and posts.

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Formstack’s <a href="https://www.formstack.com/integrations/wordpress?utm_source=wordpress.com&utm_medium=referral&utm_campaign=integrations" rel="nofollow ugc">WordPress form plugin</a> makes it quick and easy to embed contact forms, lead generation forms, payment forms, and more on your WordPress blogs and websites. Build customized online forms in seconds with our <a href="https://www.formstack.com/features/drag-and-drop?utm_source=wordpress.com&utm_medium=referral&utm_campaign=integrations" rel="nofollow ugc">drag-and-drop interface</a>, and integrate with <a href="https://www.formstack.com/integrations?utm_source=wordpress.com&utm_medium=referral&utm_campaign=integrations" rel="nofollow ugc">40+ third-party apps</a>, including Salesforce, MailChimp, and PayPal.
This plugin features two components:
<ul>
<li>Formstack Widget</li>
<li>Formstack Plugin</li>
</ul>
The Formstack widget allows you to embed Formstack web forms into your sidebar. The widget automatically optimizes the web form’s CSS to make your online forms fit and look great on your WordPress pages.
The Formstack Plugin adds a button to the TinyMCE editor that allows you to easily select the Formstack web form you wish to embed. Once a form is selected, a shortcode will be inserted into the editor, which will be converted to the selected form once your page or blog post is rendered.
This plugin supports the following shortcodes <code>[Formstack]</code>, <code>[formstack]</code>, and <code>[fs]</code>.
A Formstack account and API key are required. <a href="https://www.formstack.com/pricing?source=wp-plugin_utm_medium=pluginlisting" title="Free Online Forms" rel="nofollow ugc">Signup for free today!</a>

Formstack Online Forms

I'm not sure wrapping if this is just this version or all versions, but wrapping your site in an iframe doesn't constitute as a functioning plugin. 
Doesn't work on WP 4.5 as far as i'm concerned.



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