It is not possible to embed the review and sign view ("the signing view") on your website using an iframe. We instead recommend forwarding the party back to your website after they have signed the case, or opening the signing view in a new tab or new window.
You can also use popup windows, but many browsers have restrictions in place that makes popups hard to work with. (There are multiple different techniques to utilize popups, and some examples can be found on https://www.popupcheck.com/, a site that is not affiliated with Assently.)
It might also be good to add a some text to the signing view to inform parties what to do next after signing or forward them back to your page after signing. See the helpdesk article Can I add a text to the signing view? for more information.