How can I change the speed of spoken words, using CognitiveServices in Azure?

I want to save text to mp3, how can I change the speed of the spoken language, using CognitiveServices.Speech.SpeechSynthesizer?

System:

var synth = new System.Speech.Synthesis.SpeechSynthesizer()
synth.Rate = (int)-10;

CognitiveServices:

var synth = new Microsoft.CognitiveServices.Speech.SpeechSynthesizer(config, fileOutput)
//change speed?