Si estás en Vb.Net las declaraciones de las api's son diferentes, prueba así, ya a mi me funciono, deberia servir ._.
Imports System.Runtime.InteropServices
Public Class Form1
<DllImport("user32.dll", EntryPoint:="FindWindow", SetLastError:=True, CharSet:=CharSet.Auto)> _
Private Shared Function FindWindowByClass( _
ByVal lpClassName As String, _
ByVal zero As IntPtr) As IntPtr
End Function
<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
Private Shared Function SendMessage(ByVal hWnd As IntPtr, ByVal Msg As UInteger, ByVal wParam As String, ByVal lParam As String) As IntPtr
End Function
Private WM_SETTEXT As Long = &HC
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim ConsoleHandle As IntPtr = FindWindowByClass("ConsoleWindowClass", IntPtr.Zero)
If ConsoleHandle <> IntPtr.Zero Then
SendMessage(ConsoleHandle, WM_SETTEXT, 0, "Hello")
End If
End Sub
End Class