Being able to hide the menu bar is a common requirement for embedded portals. To set this up, you need to go to:
Extension Settings
Header section
Turn off the Show Portal Header toggle
Optionally, go to the Menu section and also turn off Show Tables and Menu Items. When this option is enabled, tables and external links will be shown as a dropdown menu when the portal header is hidden.
By default, the logout button will still be available even if this toggle is turned off. You can hide it by turning on Hide Logout Button in the Profile section.