1)you can add web form directyly in header tag web from
<mata name="Description" content="bla bla">
2)secondly code behind page
/Page description
HtmlMeta Desc = new HtmlMeta();
Desc.Name = "Description";
Desc.Content = "Abc";
Header.Controls.Add(Desc);
HtmlMeta keywords = new HtmlMeta();
keywords.Name = "keywords";
keywords.Content = "asp.net,C#.NET";
Header.Controls.Add(keywords);