Forms in Wix Studio do not have onSubmit listener

The new forms in Wix Studio don’t seem to work the way documentation for WixForms suggests. I was trying to use the onWixFormSubmit listener to manipulate some data. However, that listener was unrecognized because I found that the Wix Form is coming in as a type IFrame - which doesn’t have that method. See attached screenshot.

Any workarounds?