Вы помещаете теги SSML вокруг текста, который необходимо применить к
, например.для вашего примера
--data '{"text": "<express-as type='GoodNews'> The visceral leishmaniasis, also known as kala-azar </express-as> is characterized by irregular bouts of fever, substantial weight loss, swelling of the spleen and liver, and anaemia. Visceral leishmaniasis is fatal in over 95% of cases if left untreated."}'