I see your external link URL shows esc portal. I found the ESC portal doesn't have a form page of its own to render the incident form. And hence I specified the portal to be sp.
Also, I'm afraid its going to have to live inside a Taxonomy topic.
If you want it to stay as a menu, directly on the header, put it as a menu on the top-right of the page, where you have My Tasks, My requests, etc.
I think its called Menu Item table.
So in case you need it, first open your Portal record for esc(backend), then open its associated Main Menu, on the related list, you will see Menu Items
