test.cs

void ComposeHeader(IContainer container)
{
    var titleStyle = TextStyle.Default.FontSize(20).SemiBold().FontColor(Colors.Blue.Medium);

    container.Row(row =>
    {
        row.RelativeItem().Column(column =>
        {
            column.Item().Text($"Invoice #{Model.InvoiceNumber}").Style(titleStyle);

            column.Item().Text(text =>
            {
                text.Span("Issue date: ").SemiBold();
                text.Span($"{Model.IssueDate:d}");
            });

            column.Item().Text(text =>
            {
                text.Span("Due date: ").SemiBold();
                text.Span($"{Model.DueDate:d}");
            });
        });

        row.ConstantItem(100).Height(50).Placeholder();
    });
}