Inserting strings into XML

I've created a .net program that essentially takes in a bunch of user options, then it passes these options into a 3rd party .dll through an xmlstring. My issue im having is taking my user data and inserting into into my XMLstring. Some example of my xml,

        XmlString.Append("      <Input>");
        XmlString.Append("        <ID>ReqInp_DrivenSpeed</ID>");
        XmlString.Append("        <Value>875</Value>");
        XmlString.Append("      </Input>"); 

This Xml is provided by the 3rd party, all i can change is the value, 875. I attempted to do it like this,

        XmlString.Append("      <Input>");
        XmlString.Append("        <ID>ReqInp_DrivenSpeed</ID>");
        XmlString.Append("        <Value>" + textbox1.SelectedText.Tostring() + "</Value>");
        XmlString.Append("      </Input>");

I thought this would work as its similar to what you can do with SQL, however, I'm getting no results back. Also the value entered into the textbox is 875, as its already a value accepted by the 3rd party app.