You can create a custom Menu item that links to a specific position on a page, rather than a page within your website.
On the page, find the position you want the reader to snap to when they select that item from the Menu.
In “Text” mode, insert a page anchor eg. <a name=”pickled-onions”></a> above the text or heading you want to snap to.
Go to Appearance/Menus and create a custom menu item including the page name plus the anchor eg http://www.domain.com/testing-only#pickled-onions
That will create a link to the section Pickled Onions on the page named testing-only.
See working example here, select “Pickled Onions Test” from either of the Menus displayed.