JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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You can insert our form in the pop-up by using the iFrame embed method. Please take a look at our guide on how to get the iFrame embed code. You should then be able to insert the form in the popup as a template.
I didn't have any experience with Adobe Muse but if pop-up plugin supports adding custom HTML templates you should be able to use the method I have suggested.
Please let us know if you need any assistance.