public bool DifferentFirstPage {get; set;}

// Create a document.
using (var document = DocX.Create(@"Test.docx"))
{
   // Add header support to this document.
   document.AddHeaders();

   // The header used for the first page of this document.
   Header first = document.Headers.first;

   // Force the document to use a different header for first page.
   document.DifferentFirstPage = true;

   // Content can be added to the Headers in the same manor that it would be added to the main document.
   Paragraph p = first.InsertParagraph();
   p.Append("This is the first pages header.");

   // Save all changes to this document.
   document.Save();
}// Release this document from memory.