launching with known email address

Content

Our support site is a separate entity from our sales site and control panel.  During a sale, the customer may have given their email address during checkout and when logged into their control panel, we already know their email address.

How can I launch chat from these external sites and pass to chat the known email address without having to throw up another gated (chat_launch) page?

/app/chat/chat_launch_preauth (below) throws up a window with the email address pre-populated.  I can rn_hidden the FormInput to just leave the button.  But essentially I want to go from the [SPAC] to chat_landing