Heres another method that gets the effect youre looking for but doesnt use messagebox. it looks a lot longer but it just offers much more in terms of customization.
def popupmsg():
popup = tk.Tk()
def leavemini():
popup.destroy()
popup.wm_title("Coming Soon")
popup.wm_attributes('-topmost', True) # keeps popup above everything until closed.
popup.wm_attributes("-fullscreen", True) # I chose to make mine fullscreen with transparent effects.
popup.configure(background='#4a4a4a') # this is outter background colour
popup.wm_attributes("-alpha", 0.95) # level of transparency
popup.config(bd=2, relief=FLAT) # tk style
# this next label (tk.button) is the text field holding your message. i put it in a tk.button so the sizing matched the "close" button
# also want to note that my button is very big due to it being used on a touch screen application.
label = tk.Button(popup, text="""PUT MESSAGE HERE""", background="#3e3e3e", font=headerfont,
width=30, height=11, relief=FLAT, state=DISABLED, disabledforeground="#3dcc8e")
label.pack(pady=18)
close_button = tk.Button(popup, text="Close", font=headerfont, command=leavemini, width=30, height=6,
background="#4a4a4a", relief=GROOVE, activebackground="#323232", foreground="#3dcc8e",
activeforeground="#0f8954")
close_button.pack()