The existing edit mode dropdown in the WikiEditor toolbar needs to be customized to include machine suggestions mode. See T269653: Add a link: edit mode toggle (machine suggestions & visual) for background.
This toolbar is set up differently from the VE toolbar. It looks like VisualEditor and MobileFrontEnd are customizing the edit mode dropdown in ve.init.mw.DesktopArticleTarget.init.js and SourceEditorOverlay.js respectively.