Master branch (v23.0) doesn't have structbox?

If you’d still need a solution for your original question, according to a forum discussion (Unreal Engine 4.22 Released! - #153 by Nerdsbeware - Announcements - Unreal Engine Forums) you can just add the plugin from 4.21 to your project, rebuild it, and it should work. You’ll have to maintain it on your own though.