How do I get combobox selection to appear on excel range

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Python3.6

I am new to python and I need python combobox selection to show on an excel sheet('Header'), range('C25'). The code I am using is as follows. Any help will be highly appreciated

What I have tried:

<pre><pre lang="Python">

import tkinter as tk
from tkinter import ttk
from tkinter.messagebox import showinfo
from calendar import month_name
import xlwings as xw

class ComboboxSelectionWindow():
    def __init__(self, master):
        self.master=master
        self.master.geometry('500x250')
        self.master.style= ttk.Style()
        self.master.style.theme_use('clam')
        self.master.style.configure("TCombobox", fieldbackground= "light blue", background= "white")
        
        self.entry_contents=None
        self.labelTop = tk.Label(master,text = "Porosity Scale Selection", background = 'blue', foreground ="white", font = ("Arial", 15))
        self.labelTop.place(x = 20, y = 10, width=240, height=30)
        self.comboBox_example = ttk.Combobox(master,values=["Limestone","Sandstone","Dolomite"])
        
        self.comboBox_example.current(0)
        self.comboBox_example.place(x = 20, y = 60, width=200, height=25)

        self.okButton = tk.Button(master, text='OK',command = self.callback)
        self.okButton.place(x = 20, y = 90, width=140, height=25)

    def callback(self):
        """ get the contents of the Entry and exit
        """
        self.comboBox_example_contents=self.comboBox_example.get()
        self.master.destroy()
        
def ComboboxSelection():

    app = tk.Tk()
    app.geometry('180x100')
    Selection=ComboboxSelectionWindow(app)
    app.mainloop()

    return Selection.comboBox_example_contents
    
wb = xw.books['1. Petrophysical Analysis.xlsx']
shth = wb.sheets['Header']
wb.sheets['Header'].select()
wb.sheets['Header'].cells(25, 3).Value = ComboboxSelection()