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
Portal Header section
Turn off the Show Portal Header toggle
Optionally, also turn off Show Tables and Menu Items. When this option is enabled, tables and menu items 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.