Hello! For external forms, is there a way to conceal the URL of the form? By default, the URL comprises of [accountid].extforms.netsuite.com/app/site/hosting/script=[scriptid]&depoly=1/...

Unlikely. You’d have to do some sort of Domain Masking which is pretty shady. For good reasons, it should be hard to hide what URL you are actually on.

Old fashion HTML trick put it in a frame. Also you can do post method in your own custom HTML form page.