Hi Aamir,
To clarify what Allison was saying, out-of-the-box forms will not pre-populate if they are embedded on a website, landing page, etc. Custom code is required to make an embedded form pre-populate. Since you cannot pass parameters through an iFrame, you would not be able to pre-fill a form embedded that way; and unfortunately, we are currently experiencing an issue where our widgets are not responding to javascript.
As the help article you referenced mentions, "there are other scenarios where this can be used. Instead of building the URL and its parameters in an email template, you could build it on your website." In order to implement this code, you would need to actually host the form on your website, instead of embedding it.