Send multidimensional array as Guzzle form_params - php

Question

I'm using Laravel 9 with a PHP Linnworks API repository (https://github.com/booni3/linnworks) which uses Guzzle for the API requests like so: linnworks->api->Orders()->post($url, $parameters );
I'm trying to re-create the following CURL POST request (which works fine) with the above repository's Guzzle implementation:
curl_setopt_array($curl, array(
CURLOPT_URL => "https://eu-ext.linnworks.net//api/Orders/SetExtendedProperties",
CURLOPT_RETURNTRANSFER => true,
CURLOPT_ENCODING => "",
CURLOPT_MAXREDIRS => 10,
CURLOPT_TIMEOUT => 30,
CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
CURLOPT_CUSTOMREQUEST => "POST",
CURLOPT_POSTFIELDS => "orderId=bf812fc0-d655-ecda-26da-bd64bcb3898a&extendedProperties=[
{
\"RowId\": \"ea89429e-5c71-4069-8340-7fad55ffe056\",
\"Name\": \"Net-Total\",
\"Value\": \"123\",
\"Type\": \"PROFITCALC\"
}
]"
As you can see, the postfields are just sent as a string, which was nice and easy as Linnworks API can be a bit all over the place with the request formats, but with this Guzzle implementation I'm having to send $parameters as an array. I've tried json encoding the extendedProperties sub-array like so:
$parameters = [
"orderId" => "bf812fc0-d655-ecda-26da-bd64bcb3898a",
"extendedProperties" => json_encode([
"RowId" => "ea89429e-5c71-4069-8340-7fad55ffe056",
"Name" => "Net-Total",
"Value" => "123",
"Type" => "PROFITCALC"
])
];
$url = "Orders/SetExtendedProperties/";
$extendedPropertiesNew = $this->linnworks->api->Orders()->post($url, $parameters );
Unfortunately though, I'm getting the following error returned by the API:
Client error: `POST https://eu-ext.linnworks.net/api/Orders/SetExtendedProperties/` resulted in a `400 Bad Request` response: {"Code":null,"Message":"Invalid parameter extendedProperties"}
Which doesn't make much sense to me as the request documentation here (https://apps.linnworks.net/Api/Method/Orders-SetExtendedProperties) states extendedProperties is a legit parameter and my Curl request also works fine.
I've also tried not using json_encode and just sending $parameters as a multi dimensional array but that just returns an empty array, deletes any existing order extended properties but no extended properties are written as intended, and as my old Curl request does successfully.
If it's of any help, please see booni3/Linnworks post and parse class methods:
public function post($url = null, array $parameters = []): array
{
return $this->parse(function() use($url, $parameters){
return $this->client->post($this->server.$url, [
'form_params' => $parameters,
'headers' => [
'Content-Type' => 'application/x-www-form-urlencoded',
'Accept' => 'application/json',
'Authorization' => $this->bearer ?? ''
]
]);
});
}
private function parse(callable $callback)
{
$response = call_user_func($callback);
$json = json_decode((string) $response->getBody(), true);
if(json_last_error() !== JSON_ERROR_NONE){
throw new LinnworksResponseCouldNotBeParsed((string) $response->getBody());
}
return $json;
}
Any ideas on how to get the form_params to match exactly with my curl request so I can get this to work? I've used this repository successfully with countless other Linnworks API requests already but I'm simply stuck on this one so any help would be greatly appreciated.