PyBitmessage/src/main.kv

<LoginScreen>:
    BoxLayout:
        AnchorLayout:
            TextInput:
                id: user_input

    BoxLayout:
        Button:
            id: sendmessage
            text: 'SendMessage'
            on_press: root.send()

        Button:
            id: btnExit
            text: "Exit"
            on_press: root.sayexit()