If you use the Notes feature in Outlook, you may have noticed that it does not look the same in the new Webmail interface.
In Outlook, the notes appear in a 'post-it' style whereas Webmail notes appear like draft email messages.
From Webmail, pre-existing notes can be edited and saved to your Drafts folder, however new notes cannot be created.
As a workaround, you can consider creating notes in the Tasks folder or create an additional Tasks folder to hold all your Webmail notes.
If you decide to create notes in the existing Tasks folder, you can mark these with a Category or any other sorting or filtering method to differentiate from your regular Tasks. You can also mark them as 'Done' to prevent each note from showing up as a To-Do item or a pop-up reminder.