This is your code but with best practises:

import tkinter as tk

def create_stamp():
    stamp = tk.Label(window2, text="Stamp")
    stamp.pack()

def create_second_win():
    global window2
    window2 = tk.Toplevel(root)
    lab2 = tk.Button(window2, text="Click me", command=create_stamp)
    lab2.pack()


root = tk.Tk()
root.title("Frame control")

button = tk.Button(root, text="Open a new window", command=create_second_win)
button.pack()

root.mainloop()

I made window2 a global variable so that I can access it from create_stamp. Generally it is discouraged to use from ... import *. As @Matiiss said, sometimes you can have problems with global variables if you don't keep track of the variable names that you used.

If you want to avoid using global variables and want to use classes, look at this:

import tkinter as tk


class App:
    def __init__(self):
        self.stamps = []

        self.root = tk.Tk()
        self.root.title("Frame control")

        self.button = tk.Button(self.root, text="Open a new window", command=self.create_second_win)
        self.button.pack()

    def create_stamp(self):
        stamp = tk.Label(self.window2, text="Stamp")
        stamp.pack()
        self.stamps.append(stamp)

    def create_second_win(self):
        self.window2 = tk.Toplevel(self.root)
        self.lab2 = tk.Button(self.window2, text="Click me", command=self.create_stamp)
        self.lab2.pack()

    def mainloop(self):
        self.root.mainloop()


if __name__ == "__main__":
    app = App()
    app.mainloop()

As @Matiiss mentioned it would be more organised if you move the second window to its own class. For bigger projects it is a must but in this case you don't have to.