Как задействовать кнопку вызова функции которая находится в другой функции

from atexit import register
from tkinter import *
from tkinter import messagebox
from turtle import Screen 
#def register_user():
       


def check_password(*args, **kwargs):
    login_entry.configure(background="green" if username.get() == "admin" else "red")
    password_entry.configure(background="green" if password.get() == "admin" else "red")
    
    def register():
    
        global login_entry
        global password_entry
        global username
        global password
        
    
        screen1 = Toplevel(screen)
        screen1.geometry("300x300")
        screen1.title("Test1")
      
        username = StringVar()
        password = StringVar()
   
        Label(screen1, text = "username").pack()
        login_entry = Entry(screen1, textvariable = username)
        login_entry.pack()
        Label(screen1, text = "password").pack()
        password_entry = Entry(screen1, textvariable=password, width=8)
        password_entry.pack()
        password_entry.bind("<KeyRelease>", check_password)

        Button(screen1, text = "registr", width = 25, height = 2, command = check_password)
        Button.pack()
    return register()
check_password()
check_password(register)()
    
def login():
    messagebox.showinfo("", "Login sessiyasi baslandi")
    
    
def main_screen():
    
    screen = Tk()
    screen.geometry("300x300")
    screen.title("Test")
    Label(text = "").pack()
    Button(text = "login", width = 25, height = 2, command = login).pack()
    Label(text = "").pack()
    Button(text = "register", width = 25, height = 2, command = register).pack()        
    
    screen.mainloop()


main_screen()