You can use a legacy Smart Form, choosing the HTML Version of the code, and embed that. Note that in most accounts the HTML Version of the code is disabled in order to allow us to implement stricter anti-spam measures. But you can enable it as per these instructions. You can view these instructions on using the legacy Smart Form Builder.
For ONTRAforms, you can also use the "click to pop lightbox" version of the code. This gives you a link to insert into the page and, upon clicking the link, the Wix page visitor will get the form popping up in a lightbox.