You are posting message to a wrong window. Try this:
Imports System.Runtime.Interop
Const EM_SETINPUTMODE As Integer = &HDE
Const EM_GETINPUTMODE As Integer = &HDD
Dim wparam As New IntPtr(0)
Dim lparam As New IntPtr(2)
Dim newmsg As New Microsoft.WindowsCE.Forms.Message
Dim test As New Microsoft.WindowsCE.Forms.MessageWindow
MyEditControl.Capture = True
Dim hWnd as IntPtr = GetCapture()
MyEditControl.Capture = False
newmsg.Create(hWnd, EM_SETINPUTMODE, wparam, lparam)
test.PostMessage(newmsg)
<DllImport("coredll">_
Shared Function GetCapture() as IntPtr
End Function