https://learn.microsoft.com/en-us/dotnet/api/system.speech.synthesis.speechsynthesizer.addlexicon?view=netframework-4.8.1&viewFallbackFrom=net-10.0-pp
SpeechSynthesizer.AddLexicon(Uri, String) Method (System.Speech.Synthesis) | Microsoft Learn
Adds a lexicon to the SpeechSynthesizer object.
speech synthesisuristringmethodsystem