Creating a new Windows Forms Application Project, then open your form designer.
Next, Set the FormBorderStyle to None and BackColor to White.
Dragging TextBox, Label, Button, CheckBox and PictureBox controls from the visual studio toolbox to your winform, then design a simple modern ui login form as the following video.
To create a flat button you should set the FlatStyle property to Flat, ForeColor to White and BackColor to (15, 157, 88).
To make your form support transparent background, you can select your winform, then reduce the value of Opacity property, for example reduce 50%.