When submitting JSON files with SSML input, you can use the Prosody tag in the following format[1][2].
Example:
<prosody rate="slow" pitch="-2st">Can you hear me now?</prosody>
[1] https://cloud.google.com/text-to-speech/docs/ssml#prosody
[2] https://www.w3.org/TR/speech-synthesis11/#:~:text=3.2.4%20prosody%20Element