I have several custom tags that I use in OneNote 2013. These tags show up on my WP OneNote app, as well as on the Modern / Metro version on Windows 8.1 RT.
I've done some searching and other than finding old (circa 2010) telling me that custom tags won't sync I can't find anything showing how to get those custom tags into OneNote Modern / Metro or if it can't be done, how to create tags in OneNote Modern / Metro.