Hola!
puedes hacerlo en el evento "RowsAdded" del datagridview
Código:
Private Sub DataGrid_RowsAdded(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewRowsAddedEventArgs) Handles DataGrid.RowsAdded
DataGrid.Rows(DataGrid.CurrentRow.Index).Cells("Articulo").Value = TextBox1.Text
End Sub
Saludos!