There's a "scratch pad" on your navigation home page.
Additionally, you can create personal free-space pages; for example, I have Space:Smith-62120_-_Useful_Strings_of_Code.
To create a free-space page, go to Add on the Wikitree menu and select "New Thing." To make it clear that a page is personal, start the title with your user ID: for example, you could title it "Powell-2509 Personal Notes".