If you have installed "Gem for OneNote" add-in, you also quickly open this URL from desktop OneNote.
Open Web Page to Setting the Default Notebook Online from Desktop OneNote
In "Gem for OneNote", click "Cooperation" tab -> "Share" group -> "One Drive" menu -> "Default Online Notebook Settings" item.
Gem will navigate to https://www.onenote.com/EmailSettings.
After login to your microsoft account, you can choose which notebook for the me@OneNote.com.