This is should be done in the PlatformUI Search prototype bundle.

For the button, the default eZ.ButtonActionView can be used for that, when clicking on the button the user will be redirected on the search page (empty for now).

Technically, this should be done from 2 plugins:

• one plugin for the discovery bar view to add the "Search" button (right before "Content Tree")
• one plugin for the app to add the route (see https://doc.ez.no/display/DEVELOPER/Alter+the+JavaScript+Application+routing)