Outlook 365: Number of unread messages in a folder with sub-folders

It couldn't be directly fulfilled, but as an alternative, we could create a search folder to count the unread items in your "MyAdmin" folder (including all subfolders).

Right-click Search folders > New Search Folder > Custom > Create a custom search folder > Choose. Type a Name and select Criteria > More Choices > Only items that are "Unread". Now select Browse, untick the top-level folder (for your email account), and select your "MyAdmin" folder and check "Search subfolders".

Note that in the image, the top-level folder should be unticked!

You could also add this folder to the Favorites list to make it obvious.