from tkinter import *


def clicked():
    res = "Hallo {}".format(txt.get())
    lbl.configure(text=res)


window = Tk()
window.title("Willkommen bei der Bewerbung")
window.geometry('400x250')
lbl = Label(window, text="Hallo")
lbl.grid(column=0, row=0)
txt = Entry(window, width=20)
txt.grid(column=1, row=0)
btn = Button(window, text="Klicken!", command=clicked)
btn.grid(column=2, row=0)
window.mainloop()