import tkinter
window = tkinter.Tk()
x=None
ent = tkinter.Entry(window)
ent.bind("<Return>", lambda x: ent.get())# вот, собственно, и вопрос

import tkinter as tk

window = tk.Tk()
x = []
ent = tk.Entry(window)
ent.v = tk.StringVar()
ent['textvariable'] = ent.v
ent.pack()
ent.bind("<Return>", lambda key: x.append(ent.v.get()))