To create a new custom tag, click the Tags dropdown in the Home tab and select “Create New Tag.” When the Create a Tag pane opens, name the new tag, select an icon to represent it and click Create to add your new tag to the list of tags. Starting in January 2019, it will be available on OneNote for Windows 10 and OneNote for Mac. No settings need to be enabled to use this feature. Once you see it in your app, feel free to start using it. OneNote for Windows 10 and Mac finally gets Custom Tags in 2019.
