I am getting Response code 400 with the following POST connection in Kotlin using java.net standard library shipped in Kotlin and kotlinx.serialization.json API. The same works on a online API Testing tool, hence I suspect I am doing something wrong in sending the payload.

val con = URL("https://thirdparty-api-service.com/token").openConnection() as HttpURLConnection
con.requestMethod = "POST"
con.setRequestProperty("Content-Type", "application/x-www-form-urlencoded")
val myClientId : String = <my_client_id>
val myClientSecret : String = <my_client_secret>
val data = buildJsonArray {
    addJsonObject {
        put("grant_type", "client_credentials")
        put("client_id", myClientId)
        put("client_secret", myClientSecret)
    }
}
Log.i(LOG_TAG,"Sending data: $data")
con.doOutput = true
con.outputStream.bufferedWriter().use {
    it.write(data.toString().toCharArray())
    it.flush()
}
Log.i(LOG_TAG,"Response Code: ${con.responseCode}")
con.disconnect()

The above logs the following:

Sending data: [{“grant_type”:”client_credentials”,”client_id”:”my_client_id”,”client_secret”:”my_client_secret”}]

Note that in the testing tool I used the following format for sending the payload:

grant_type=client_credentials

client_id=my_client_id

client_secret=my_client_secret