MercatorUi.GridPro.TopHeaders topHeaders = new MercatorUi.GridPro.TopHeaders(grid)
{
    Items = new MercatorUi.GridPro.TopHeader[]
    {
        new MercatorUi.GridPro.TopHeader { ColumnStart = 0 },
        new MercatorUi.GridPro.TopHeader { ColumnStart = 1, ColumnSpan = 2, Text = "Document" },
        new MercatorUi.GridPro.TopHeader { ColumnStart = 3, ColumnSpan = 2 },
        new MercatorUi.GridPro.TopHeader { ColumnStart = 5, ColumnSpan = 3, Text = "Totaal" },
        new MercatorUi.GridPro.TopHeader { ColumnStart = 8 }
    }
};
topHeaders.Build();