There are many form builders (such as wufoo, google forms, and typeform) that allow you to create and embed forms into your site. Unfortunately, since these form builders utilize iframes, Tend is not able to capture the data from these forms.
The reason is the Same Origin Policy, which stops Javascript from accessing content within an <iframe>, if that <iframe> is from a different domain. The Same Origin Policy in place to protect us all from malicious activity.
With that said, there are some workarounds.
Some form builders will allow you to download the HTML for the form. By adding the form's HTML to your site, Tend will be able to automatically capture the visitor's information. Here is an example of where you can download the HTML through Wufoo.
Another option is to redirect visitors "Thank You" page with Tend installed, after they completed the form. The trick is to include the visitor's email as a URL parameter. Not all form builders allow for this, but here is how you would setup the redirect with Wufoo, and with Typeform.
You can send Tend the contact's email address you get from the software that collects the email address and push it to Tend while the user is on the site. Tend will then match that email address to the visitor's activity and populate their contact information.
This can be done by using TendID or by using Tend's Zapier integration.