How to add text to request body in RestSharp

Here is how to add plain xml string to the request body:

req.AddParameter("text/xml", body, ParameterType.RequestBody);

To Add to @dmitreyg's answer and per @jrahhali's comment to his answer, in the current version, as of the time this is posted it is v105.2.3, the syntax is as follows:

request.Parameters.Add(new Parameter() { 
    ContentType = "application/json", 
    Name = "JSONPAYLOAD", // not required 
    Type = ParameterType.RequestBody, 
    Value = jsonBody
});

request.Parameters.Add(new Parameter() { 
    ContentType = "text/xml", 
    Name = "XMLPAYLOAD", // not required 
    Type = ParameterType.RequestBody, 
    Value = xmlBody
});