Changing the container of a custom form

Question

I made a custom form, with the style of Windows 10, but there is a problem, it is considering the caption bar as part of the form client panel.

This is the form. The problem is that, the position 0, 0 of the form is inside Caption, where it could not be, should be below Caption; follow the example below.

So how do I modify the client area of the window? (Area where the controls can be put)