from tkinter import *
root=Tk()

def fcIn():
    e.delete(0, END)
def fcOut():
    e.insert(0, 'nome')

l = Label(root,text="User name")
l.pack()

e= Entry(root)
e.pack()
e2= Entry(root)
e2.pack()

fcOut()
e.bind("<FocusIn>", fcIn)
e.bind("<FocusOut>", fcOut)

root.mainloop()