Desde Form Producto con este evento llamo a form Stock:
 
Código:
 Private Sub btnNuevo_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNuevo.Click
        Dim frmStock As frmStock = frmStock.Instance
        'Parametro del form 
        frmStock.Opcion = "Nuevo"
        'Muestro el form
        frmStock.ShowDialog()
End Sub
  
El codigo de la Form Stock: 
Código:
 Public Class frmStock
    'Variables Globales
    Public CodigoStock As Integer = Nothing
    Public Opcion As String = Nothing
    'Variable pública.
    Private Shared ChildInstance As frmStock = Nothing
    'controla que sólo exista una instancia del formulario.
    Public Shared Function Instance() As frmStock
        If ChildInstance Is Nothing OrElse ChildInstance.IsDisposed = True Then
            ChildInstance = New frmStock()
        End If
        ChildInstance.BringToFront()
        Return ChildInstance
    End Function
    Public Sub New()
        ' This call is required by the Windows Form Designer.
        InitializeComponent()
        ' Add any initialization after the InitializeComponent() call.
    End Sub
   
    Private Sub btnGuardar_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnGuardar.Click
       
    End Sub
End Class