I am using Python 3.11.9 on windows OS.
I want to create button in tkinter which when pressed add text to entry i selected before clicking the button.
Here is my example code

from tkinter import *
class Application(Frame):
    def __init__(self, master=None):
        Frame.__init__(self, master)
        frtop=self.winfo_toplevel() #Flexible Toplevel of the window
        screen_width = self.winfo_screenwidth()
        screen_height = self.winfo_screenheight()
        print("width="+str(screen_width)+", height="+str(screen_height))
        max_windows_size=str(screen_width)+"x"+str(screen_height)
        frtop.geometry(max_windows_size)
        self.grid()
        self.cash_frame = Frame(self)
        self.cash_frame.grid(column =0, row =0, sticky='NSEW',columnspan=1)
        self.cash_lbl = Label(self.cash_frame, text = "Cash:", font=('Arial',20,'bold'))
        self.cash_lbl.grid(column =0, row =0)
        self.cash_str_null = StringVar() 
        self.cash_str_null.set('')
        self.cash = Entry(self.cash_frame, width=5,textvariable=self.cash_str_null, font=('Arial',26,'bold'))
        self.cash.grid(column =1, row =0)
        self.exchange_frame = Frame(self)
        self.exchange_frame.grid(column =0, row =1, sticky='NSEW')
        self.lbl_exchange = Label(self.exchange_frame, text = "exchange",font=('Arial',20,'bold'))
        self.lbl_exchange.grid(column =0, row =0)
        self.exchange_str_null = StringVar() 
        self.exchange_str_null.set('')
        self.exchange_value = Entry(self.exchange_frame, width=5,textvariable=self.exchange_str_null,font=('Arial',20,'bold'))
        self.exchange_value.grid(column =1, row =0)
        self.add_one = Button(self, text = "1" , fg = "red", command=self.add_one_in_entry,font=('Arial',20,'bold'))
        self.add_one.grid(column=0, row=2)

    def add_one_in_entry(self):
        pass

if __name__ == "__main__":
    app = Application()
    app.master.title("Sample application")
    app.mainloop()

How can I do that ?

Thanks