rectangularFlatButtonKivymd.py

from kivymd.app import MDApp
from kivymd.uix.button import MDRectangleFlatButton
from kivymd.uix.screen import MDScreen


class Main(MDApp):
    def build(self):
        screen = MDScreen()
        btn = MDRectangleFlatButton(text="Hello World",
                                    pos_hint={'center_x': 0.5, 'center_y': 0.5}
                                    )
        screen.add_widget(btn) 
        return screen


Main().run()