Embedding html log in form to lightbox

Hi All,
I’m looking to embed my created html login/signup form into Wix. I’ve tried placing the HTML element into the lightbox but was wondering how to send the login/signup information from the HTML element into Wix’s user database. I’ve tried the messaging demonstration between site code and HTML but they mainly deal with an already created element on Wix to receive the information. Does anyone know if this is possible and if so, how would I approach it? Thanks!