self.box = ttk.Combobox(self.parent, textvariable=self.box_value, state=
    'readonly')
self.box['values'] = 'A', 'B', 'C'
self.box.current(0)
self.box.grid(column=0, row=0)
if __name__ == '__main__':
    root = Tk()
app = Application(root)
root.mainloop()
Set a default value for a ttk Combobox