take a look in Elements/Interaction, you'll find Form Container where you can add 'input' elements.
If you don't know php but use Web Form Builder and want use its Confirm Email, Confirm Page, Notification, etc.
First create rapidly your form prototype in WFB with elements needed, no page layout but only note all elements names and values. Fill settings. Save (e.g. "Myform.fb) and export 'manual server setup'(1).
WFB create a folder 'Myform' with php files in, and a file Myform.php.
Put those folder and file somwhere accessible on server (maybe in a 'Forms' folder same level as SD3 pages).
Then go to SD3 and create the form with same elements names and values, form can be set in any SD3 page, using flex or grid.
Finally, in SD3, select the Form container in elements tree and simply put 'the path to Myform.php file on server' in the Action field (e.g. https://mysite.com/Forms/Myform.php).
Save, export, open your page in browser... Tadaa! It works! No miracle here, Action wants php code...
No more annoying iframes export/import with html element and scripts(2)!
Ok, I know Form Designer have superior tools to create forms so iframes exp/imp stay useful (2)...
1: Sorry I do not use SDrive, so here its for use with own server.
2: cf https://www.coffeecup.com/help/articles/hook-in-a-form-builder-script-to-form-designer/