User32 sendmessage works only for some messages - vb.net

Question

I am looking to send key and mouse events to notepad.exe, however not necessarily have the window be active. In some cases the key events are actual typing operations and in other cases the key input represents shortcut key combinations.
Using SendKeys works, but obviously needs focus. I would prefer to have that operate in the background so am investivating SendMessage from user32.dll. I can see it has different signatures based on what the message code is, and this may be the origin of the problem.
Starting with
Dim notepad() As Process = Process.GetProcessesByName("notepad")
Dim actualWindow As IntPtr = FindWindowEx(notepad(0).MainWindowHandle, IntPtr.Zero, "Edit", Nothing)
and using
<System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint:="SendMessageA")>
Public Function SendMessage(ByVal hWnd As IntPtr, ByVal wMsg As Int32, ByVal wParam As Int32, ByVal lParam As String) As IntPtr
End Function
if I send
SendMessage(actualWindow, KeyEventType.WM_SETTEXT, 0, "Text")
then the editor text is replaced with "Text".
If however I use
<System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint:="SendMessageA")>
Public Function SendMessage(ByVal hWnd As IntPtr, ByVal wMsg As Int32, ByVal wParam As Int32, ByVal lParam As Int32) As IntPtr
End Function
and send
SendMessage(actualWindow, KeyEventType.WM_KEYDOWN, &H4C, 0)
SendMessage(actualWindow, KeyEventType.WM_KEYUP, &H4C, 0)
or even
SendMessage(notepad(0).MainWindowHandle, KeyEventType.WM_KEYDOWN, &H4C, 0)
SendMessage(notepad(0).MainWindowHandle, KeyEventType.WM_KEYUP, &H4C, 0)
where &H4C corresponds to the 'L' key (1.), no letter 'l' appears in the editor.
I am moderately convinced that the function signatures are correct for both WM_KEYDOWN/WM_KEYUP (2.) and WM_SETTEXT (3.). I think the problem might be one of 'focus' but am not sure how to tackle that if I want the operations to occur in the background. How can keystrokes be sent to the text editor?
The enumerations are defined as:
Public Enum KeyEventType As Int32
WM_SETTEXT = &HC
WM_KEYDOWN = &H100
WM_KEYUP = &H101
End Enum