import gdk.Keysyms; //keys enums are defined here

private void func(Button sender)
{
    //button pressed
}

but.addOnClicked(&func);

private bool func2(GdkEventKey* ev, Widget sender)
{
    if(ev.keyval == GdkKeysyms.GDK_Tab)
        return true; //we handle Tab ourselves and prevents default behaviour
    else
        return false; //we didnt handle it so gtk does default behaviour
}

win.addOnKeyPress(&func2);