param is missing or the value is empty laravel - php

Question

I am creating a user with the api provided. I am using Laravel and trying to store data to smartrmail and docs to create new subscriber is here https://docs.smartrmail.com/en/articles/636615-list-subscribers
Each time i send request i get following error:
Server error: `POST https://go.smartrmail.com/api/v1/lists/1sptso/list_subscribers` resulted in a `500 Internal Server Error` response: {"error":"param is missing or the value is empty: subscribers"}
{"error":"param is missing or the value is empty: subscribers"}
I am using Laravel and my code is here
Route::get('smartrmail',function(){
$headers = [
'Accept' => 'application/json',
'Authorization' => 'token f91715d5-3aac-4db3-a133-4b3a9493a9a4',
'Content-Type' => 'application/json',
];
$client = new GuzzleHttp\Client([
'headers' => $headers
]);
$data = [
"subscribers"=>[
[
"email"=> "vanhalen#example.com",
"first_name"=> "van",
"last_name"=> "halen",
"subscribed"=> true,
]
]
];
$res = $client->request('POST', 'https://go.smartrmail.com/api/v1/lists/1sptso/list_subscribers', [
'form_params' => [
$data
]
]);
return($res);
// echo $res->getStatusCode();
});
Anybody help me to figure out what is wrong here. I am following this docs
https://docs.smartrmail.com/en/articles/636615-list-subscribers
to create a new subscriber

Answer 1

Instead of
'form_params' => [
$data
]
use
'json' => $data
Explanation
You want to send json data (I assume that because you set header 'Content-Type' => 'application/json', which means that you are sending json), but form_params is used for application/x-www-form-urlencoded.
json sets header to application/json and sends data as json.
As you set proper header, this should work too:
'body' => $data
Proper name of param you can find in Guzzle docs, I used uploading data part.