how to flag a photo in Photos

Do I have to make a smart album and use 'flagged' as a key work? Very cumbersome!!!

The keyword "flag" will not be visible on the thumbnail of the photo. The only label you can see as a badge on the photo is the "Favourites" heart. You could try to misuse this mark for flagging.

Otherwise, a flag keyword is the only option. You could add it to the quick group in the Keyword Manager, so you can flag photos quickly this way.

For example, this way you can simply type "f" to flag a selected photo.