The only place that I know of that Quick Base natively supports <iframe> is on a dashboard. Rich Text fields don't support the <iframe> tag. If you need it to appear on the form your best bet is to use the Image OnLoad technique and use it to script your own HTML into part of the page. There is an article by Dan Diebolt from a while back that illustrates how to implement an <iframe> and I copied his original set up post on how to set up IOL in a form to take a look at as well.
iFrames:
https://community.quickbase.com/quickbase/topics/embedding-an-iframe-within-a-quickbase-formIOL setup:
https://community.quickbase.com/quickbase/topics/how-do-i-setup-the-image-onload-technique-iol
