How can I write to a XML file like this format please

You can use Xelement, see example here: Deep C# - XML in C#[^]

You can save an Xelement like this:

XElement root = new XElement("Root",  
    new XElement("Child", "child content")  
);  
root.Save("Root.xml");  

Note that things like stid="-1" are called "attributes", see the example on how to create attributes for an Xelement.