using RestSharp;


var options = new RestClientOptions("https://api.sandbox.{id}.gr4vy.app/transactions/{transaction_id}/session");
var client = new RestClient(options);
var request = new RestRequest("");
request.AddHeader("Authorization", "Bearer <token>");
request.AddJsonBody("{\n  \"payload\": {\n    \"key\": \"value\"\n  }\n}", false);
var response = await client.PostAsync(request);

Console.WriteLine("{0}", response.Content);

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.sandbox.{id}.gr4vy.app/transactions/{transaction_id}/session"

	payload := strings.NewReader("{\n  \"payload\": {\n    \"key\": \"value\"\n  }\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://api.sandbox.{id}.gr4vy.app/transactions/{transaction_id}/session")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"payload\": {\n    \"key\": \"value\"\n  }\n}")
  .asString();

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.sandbox.{id}.gr4vy.app/transactions/{transaction_id}/session",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    'payload' => [
        'key' => 'value'
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Bearer <token>",
    "Content-Type: application/json"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

import requests

url = "https://api.sandbox.{id}.gr4vy.app/transactions/{transaction_id}/session"

payload = { "payload": { "key": "value" } }
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.text)

const options = {
  method: 'POST',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({payload: {key: 'value'}})
};

fetch('https://api.sandbox.{id}.gr4vy.app/transactions/{transaction_id}/session', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

curl --request POST \
  --url https://api.sandbox.{id}.gr4vy.app/transactions/{transaction_id}/session \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "payload": {
    "key": "value"
  }
}
'

{
  "session_data": {
    "key": "value"
  },
  "default_completion_url": "<string>"
}

{
  "type": "error",
  "code": "bad_request",
  "status": 400,
  "message": "Generic error",
  "details": []
}

{
  "type": "error",
  "code": "unauthorized",
  "status": 401,
  "message": "No valid API authentication found",
  "details": []
}

{
  "type": "error",
  "code": "forbidden",
  "status": 403,
  "message": "Generic error",
  "details": []
}

{
  "type": "error",
  "code": "not_found",
  "status": 404,
  "message": "The resource could not be found",
  "details": []
}

{
  "type": "error",
  "code": "method_not_allowed",
  "status": 405,
  "message": "Method Not Allowed",
  "details": []
}

{
  "type": "error",
  "code": "duplicate_record",
  "status": 409,
  "message": "Generic error",
  "details": [],
  "resource_id": "cdc70639-cb9c-4222-a73f-b8ce39f7821b"
}

{
  "detail": [
    {
      "loc": [
        "<string>"
      ],
      "msg": "<string>",
      "type": "<string>",
      "input": "<unknown>",
      "ctx": {}
    }
  ]
}

{
  "type": "error",
  "code": "too_early",
  "status": 425,
  "message": "Generic error",
  "details": []
}

{
  "type": "error",
  "code": "too_many_requests",
  "status": 429,
  "message": "Generic error",
  "details": []
}

{
  "type": "error",
  "code": "server_error",
  "status": 500,
  "message": "Request could not be processed",
  "details": []
}

{
  "type": "error",
  "code": "bad_gateway",
  "status": 502,
  "message": "Request could not be processed",
  "details": []
}

{
  "type": "error",
  "code": "gateway_timeout",
  "status": 504,
  "message": "Request could not be processed",
  "details": []
}