I am not sure what you mean by "selection box". Can you provide a screenshot? Also, what version of Windows 10 are you using? You can find out by typing in "Winver.exe" in the run dialog box.
I think I see the reason for your issue. Windowblinds works best when it is used along with Start11. While you can still use Open Shell, you are missing out on some cool features, and you may need to purchase and install Start11 in order to be able to do what you want. Start11 will also work on Windows 10. I myself used to use Open Shell, formerly Classic shell, but in order to have and to be able to do the custom things that I wanted, I felt that I needed to acquire and use Start11. As far as I know, that is the best answer I can give you. There may be a workaround, however, for your issue that I am not aware of.
