from tkinter import *
from tkinter import messagebox
def callAbout():
    messagebox.showinfo(title = "About", message = "My Window")

win = Tk()

win.geometry('300x300')
win.title("My First Window")

bar_menu = Menu(win)

menu_about = Menu(bar_menu, tearoff = 0)
bar_menu.add_cascade(label = "About", menu = menu_about)
menu_about.add_command(label = "About", command = callAbout)

win.config(menu = bar_menu)

win.mainloop()
