A family of Microsoft word processing software products for creating web, email, and print documents.
I have no idea which menu you cannot find but the Customize Keyboard command is listed at the bottom of the Tools menu... assuming you're using the desktop software.
If using the Word Online web app keyboard customization is literally non-existent, but the conventional shortcuts work as expected:
- Subscript - Shift+command+➖
- Superscript - Shift+Command+=
It's also possible that you may be accustomed to using a Windows PC. In that case the commands are somewhat different:
- Subscript - Control+=
- Superscript - Control+Shift+✚
If you continue to have a problem please provide an explicit description of what you need. Also, include the specific Version Number of your Office software & edition of macOS.