If you use Windows 10, you can open Cortana from the Classic Start Menu Search box without having to re-type your search query. Meaning you type something first into the Classic Start Menu's search. If you want Cortana to find it instead, you just press a button in the menu to pass on that query to Cortana.
The procedure for this is exactly the same as adding any other Search Provider. To do this:
1. Open Classic Start Menu Settings.
2. Go to the Customize Start Menu tab.
3.
If you are using Windows 7 style, add a custom item by pressing the Insert key or right click in an empty area of the table rows and select Insert New Item. Choose Custom there.
OR
If you use the Classic style, go to Customize Start Menu tab and scroll down in the right-side column. Drag the Custom item from right column and drop it on the "Search Box" in the left column.
4. Double click the icon of the Custom item.
5. In the Command: field, type or copy-paste the following:
ms-cortana://StartMode=MyStuff&q="%2"
6. For the Label of this button, type anything you want: e.g. Cortana or "Ask Cortana" or "Search using Cortana...".
7. For the icon, you can download and use the following Cortana icon attached to this post.
8. Click OK everywhere to save settings.
Now whenever you open Classic Start Menu and type something in its search box, you will see the Cortana item at the bottom. Click it or select it and press Enter and Cortana will open with your Classic Start Menu's search terms automatically filled.
But of course, since
Classic Shell's search is superior and far more comprehensive, we see no reason to use Cortana for
finding stuff.
You can use this feature not just for Cortana but for passing on your search box's queries to any other app. Look at the "Search Providers" section of the Start Menu Search Guide.