length = GetWindowText(hwnd_Ventana, Caption_Ventana, Len(Caption_Ventana))

Public Class Form1
    Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As Long, ByVal lpString As String, ByVal cch As Long) As Long
    Private Declare Function GetWindowTextLength Lib "user32" Alias "GetWindowTextLengthA" (ByVal hwnd As Long) As Long
    Private Function findPartialTitle(ByVal partialTitle As String) As IntPtr
        For Each p As Process In Process.GetProcesses()
            If p.MainWindowTitle.IndexOf(partialTitle, 0, StringComparison.CurrentCultureIgnoreCase) > -1 Then
                '
                'We found a match, so return the handle to the window
                '
                Return p.MainWindowHandle
            End If
        Next

        '
        'If no match, return NULL (or close enough)
        '
        Return IntPtr.Zero
    End Function

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim hwnd As Long = findPartialTitle("YouTube")
        Dim lngLen As Integer = 512
        Dim MyStr As String
        'Create a buffer
        MyStr = 255

        'Get the window's text
        GetWindowText(hwnd, MyStr, 254)
        MsgBox("La ventana abierta no es correcta, intentalo de nuevo: " &MyStr)
    End Sub
End Class

GetWindowText(hwnd, MyStr, 254)