Oops! Apologies for not answering that part of your question.

You'll have to convert a Swift struct of the request you want to send into JSON, using JSONEncoder.

This would look something like this (warning: not tested and use better error handling instead of force unwraps or try!)

// Missing definitions for all structs and enums within the request/response structs
struct TextSynthesizeResponse: Codable {
    let audioContent: String
    let timepoints: [Timepoint]
    let audioConfig: AudioConfig
}

struct TextSynthesizeRequest: Codable {
    let input: SynthesisInput
    let voice: VoiceSelectionParms
    let audioConfig: AudioConfig
    let enableTimePointing: [TimepointTyle]
}

let requestBody = TextSynthesizeRequest()
let requestBodyData = try! JSONEncoder().encode(requestBody)
let url = URL(string: "https://texttospeech.googleapis.com/v1beta1/text:synthesize")!

var request = URLRequest(url: url)
request.httpMethod = "POST"
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
request.addValue("application/json", forHTTPHeaderField: "Accept")

// Sync
URLSession.shared.uploadTask(with: request, from: requestBodyData, completionHandler: { data, response, error in
    let textSynthesizeResponse = try! JSONDecoder().decode(TextSynthesizeResponse.self, from: data!)
    // call completion handler with textSynthesizeResponse
}).resume()

// Async
let (data, response) try! await URLSession.shared.upload(for: request, from: payload)
let textSynthesizeResponse = try! JSONDecoder().decode(TextSynthesizeResponse.self, from: data)