var myCustomProperty = "MyCustomProperty";
foreach (Microsoft.Office.Interop.Word.Section section in Document.Sections)
{
    var headerRange = section.Headers[Word.WdHeaderFooterIndex.wdHeaderFooterPrimary].Range;
    foreach(Word.Field field in headerRange.Fields)
    {
        if(field.Type == Word.WdFieldType.wdFieldDocProperty
         && field.Code.Text.Contains(myCustomProperty))
        {
            //already has the header
            break;
        }
    }
    headerRange.Collapse(Word.WdCollapseDirection.wdCollapseStart);
    var f = (Word.Field)headerRange.Fields.Add(headerRange,
                           Word.WdFieldType.wdFieldDocProperty,
                           myCustomProperty,
                           true);

    f.Code.Font.Name = this.FontName;
    f.Code.Font.Size = this.FontSize;
    f.Code.Font.Bold = (int)this.IsBold;
    f.Code.Font.Italic = (int)this.IsItalic;
    f.Code.HighlightColorIndex = Word.WdColorIndex.wdYellow;
    f.Update();
    f.Code.ParagraphFormat.Alignment = Word.WdParagraphAlignment.wdAlignParagraphRight;
    f.Code.InsertParagraphAfter();
}