You can add a custom 3rd party form to your website using the iFrame widget.
Go to the page you would like to add the form to
Click Add section
Select the "Third Parties" tab on the left-hand side
Choose the iFrame Widget
Once the widget is added to the page, you can paste in the external link into the "iFrame Source" field.
For example, if you wanted to copy in a Google form, you will need to get the external share link, then paste it into the iFrame widget in Zenu.