Where to setting the default notebook for me@OneNote.com?

You can navigate https://www.onenote.com/EmailSettings to setting the default notebook for me@OneNote.com

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.

Setting the Default Notebook Online
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.