Private Declare Function WindowFromPoint Lib "user32" (ByVal xPoint As Long, ByVal yPoint As Long) As Long
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Private Declare Function GetCursorPos Lib "user32" (lpPoint As POINTAPI) As Long
Private Type POINTAPI
x As Long
y As Long
End Type
Private Const EM_SETPASSWORDCHAR = &HCC

Private Sub Form_Load()
Timer1.Interval = 100
End Sub

Private Sub Timer1_Timer()
Dim pi As POINTAPI
GetCursorPos pi
Dim wh As Long
wh = WindowFromPoint(pi.x, pi.y)
Me.Caption = "Текущий handle : " & wh
SendMessage wh, EM_SETPASSWORDCHAR, 0, 0
End Sub