using (RichEditDocumentServer wordProcessor = new RichEditDocumentServer())
Document document = wordProcessor.Document;
    Section firstSection = document.Sections[0];

    // Check whether the document already has a header (the same header for all pages).
    if (!firstSection.HasHeader(HeaderFooterType.Primary))
    {
        //If not, create an empty header.
        SubDocument headerDocument = firstSection.BeginUpdateHeader();
        headerDocument.Paragraphs.Append();
        firstSection.EndUpdateHeader(headerDocument);
    }
}

Using wordProcessor As New RichEditDocumentServer()
Dim document As Document = wordProcessor.Document
    Dim firstSection As Section = document.Sections(0)

    ' Check whether the document already has a header (the same header for all pages).
    If Not firstSection.HasHeader(HeaderFooterType.Primary) Then
        'If not, create an empty header.
        Dim headerDocument As SubDocument = firstSection.BeginUpdateHeader()
        headerDocument.Paragraphs.Append()
        firstSection.EndUpdateHeader(headerDocument)
    End If
End Using

Document document = server.Document;
document.AppendSection();
Section firstSection = document.Sections[0];
// Modify the header of the HeaderFooterType.First type.
SubDocument myHeader = firstSection.BeginUpdateHeader(HeaderFooterType.First);
DocumentRange range = myHeader.InsertText(myHeader.CreatePosition(0), " PAGE NUMBER ");
Field fld = myHeader.Fields.Create(range.End, "PAGE \\* ARABICDASH");
myHeader.Fields.Update();
firstSection.EndUpdateHeader(myHeader);
// Display the header of the HeaderFooterType.First type on the first page.
firstSection.DifferentFirstPage = true;

Dim document As Document = server.Document
document.AppendSection()
Dim firstSection As Section = document.Sections(0)
' Modify the header of the HeaderFooterType.First type.
Dim myHeader As SubDocument = firstSection.BeginUpdateHeader(HeaderFooterType.First)
Dim range As DocumentRange = myHeader.InsertText(myHeader.CreatePosition(0), " PAGE NUMBER ")
Dim fld As Field = myHeader.Fields.Create(range.End, "PAGE \* ARABICDASH")
myHeader.Fields.Update()
firstSection.EndUpdateHeader(myHeader)
' Display the header of the HeaderFooterType.First type on the first page.
firstSection.DifferentFirstPage = True