Hi Elsa,
For this kind of problems, you can use the DocumentReflector tool with SDK to see how to generated the correct C# code of the document.
Firstly, you create the document you want in Word application. Then use the tool to open the file and reflect the code for you.
For your case, you need some code look like: (supppose you want to insert a hyperlink in the 1st paragraph)
Paragraph para = document.MainDocumentPart.Document.Descendants<Paragraph>().First();
Hyperlink link = new Hyperlink(
new Run(
new RunProperties(
new RunStyle() { Val = "Hyperlink" }),
new Text("baidu")
) { RsidRunProperties = new HexBinaryValue() { Value = "002A46DA" } }
) { History = BooleanValues.One, Id = "rId4" };
para.AppendChild<Hyperlink>(link);
Besides, could you tell me more about your user scenario? what's the document will look like? and how will the result documents be used typically? Thanks:)