w:footnote (Footnote Definition)

<!-- Regular footnote with reference mark and text -->
<w:footnote w:id="1">
  <w:p>
    <w:pPr><w:pStyle w:val="FootnoteText"/></w:pPr>
    <w:r>
      <w:rPr><w:rStyle w:val="FootnoteReference"/></w:rPr>
      <w:footnoteRef/>
    </w:r>
    <w:r>
      <w:t xml:space="preserve"> ECMA-376, 5th Edition, Part 1.</w:t>
    </w:r>
  </w:p>
</w:footnote>

using DocumentFormat.OpenXml.Wordprocessing;

var footnote = new Footnote { Id = 1 };
var para = new Paragraph();
para.AppendChild(new ParagraphProperties(
    new ParagraphStyleId { Val = "FootnoteText" }
));

var refRun = new Run();
refRun.AppendChild(new RunProperties(
    new RunStyle { Val = "FootnoteReference" }
));
refRun.AppendChild(new FootnoteReferenceMark());
para.AppendChild(refRun);

var textRun = new Run();
textRun.AppendChild(new Text(" ECMA-376, 5th Edition, Part 1.")
{
    Space = SpaceProcessingModeValues.Preserve
});
para.AppendChild(textRun);
footnote.AppendChild(para);