For Windows the code is
Dim l,w,t,h As Integer
l = Self.Left
t = Self.top
h = Self.Height
w = Self.Width
Declare Function SetWindowPos Lib "User32" (ByVal hwnd As Int32, ByVal hWndInsertAfter As Int32, ByVal x As Int32, ByVal y As Int32, ByVal cx As Int32, ByVal cy As Int32, ByVal uFlags As Int32) As Int32
Const HWND_BOTTOM = 1
Const HWND_NOTOPMOST = -2
Const HWND_TOP = 0
Const HWND_TOPMOST = -1
Call SetWindowPos(Self.Handle, HWND_TOPMOST, l, t,w,h,0)
where you put this will depend - it currently assumes this is in a method or event of whatever window you want to make top most (hence the use of SELF)
You might put it in the open even of whatever window you want to make top most
and it will stay there