Set ShowWindow property of the form to 2 (top-level form). In more accurate words, WS_EX_LAYERED extended style cannot be used for child windows, as stated in Window Features aticle on MSDN.
Hit testing of a layered window is based on the shape and transparency of the window. This means that the areas of the window that are color-keyed or whose alpha value is zero will let the mouse messages through.
However, if the layered window has the WS_EX_TRANSPARENT extended window style, the shape of the layered window will be ignored and the mouse events will be passed to other windows underneath the layered window.