Tkinter project, how to edit a new window

I’ve recently learned the basics in Tkinter for a project I’m making. I’ve done a main menu with buttons that work the way I want them to. However, when I click one button that opens another window, the window is empty. How do I make the new window functional? Can I insert a class or likewise there? I want the new window to contain a drop down box and thereafter insert boxes that can store and use the information.