How do I actually user ModelView._button

I read the documentation for model view._buttons but honestly i dont know how to use it.

I am using it like this is this right.

    @classmethod
    @ModelView.button
    def on_change_with_face_reg(self, name=None):       
        return 'http://localhost:8080/face-camera'

I want to send employee_id or emplouyee information to my web app, so after they klick the button or the fields.Function they sent their information to this app.

This is how i decllare fields.fucntion()


    face_reg_web = fields.Function(fields.Char('Initialize Face'),
        'on_change_with_face_reg')

And this is how I declare XML:


    <field name="face_reg_web" widget="url"/>