How can I insert line breaks to reworded commit messages during git interactive rebase?

I want to insert line breaks in the middle of commit messages while rewording them in interactive rebase. How can I achieve this?

I'm using Notepad++ as the external editor for interactive rebase.

Simply replace pick by reword or (r for short). After you save and exit, you'll get another editor for each commit that you marked for rewording. The one-line commit message you see in the initial editor is for reference only; any changes to it seem to be ignored.