Hello - is it possible to only display certain fields on a form if the visitor is already known to us? Or vice versa, only display a field if they are not known?
It is possible by using JS to pre-check the known visitor endpoint, but let me suggest a different method that simply reuses the built-in Known Visitor HTML ("If Known Visitor, show Custom HTML") feature.
Set up 2 forms, Form A for anonymous sessions (= unknown visitors) and Form K for associated sessions (= known visitors).
Include the scaffolding for both forms (<form id="mktoForm_nnn">) in the page.
Enable KV HTML on Form A. In the Custom HTML Editor, have a <script> that loads Form K.
This way, when KV HTML is triggered, it loads the other form instead.