Adding new contact

[AshmitW]

Hey,
I've been trying to add contacts Using the API but I've been facing issues in it, if someone could guide me in what I am doing wrong it would be helpful.
Here is what I've written

$mj = new \Mailjet\Client($apikey, $apisecret, true, ["version" => "v3"]);
$body = [
"IsExcludedFromCampaigns" => "true",
"email" => "ashmit@xyz.com",
"firstname" => "Ashmit",
"lastname" => "Shetty",
"phone" => "123456",
"gender" => "M",
"birthday" => "22/09/1997",
"dexcom" => "Dummy",
"zugehoerigkeit" => "Dummy"
];
$response = $mj->post(Resources::$Contact, ["body" => $body]);
$response->success() && var_dump($response->getData());

But when I return $response->getData(), I always get this:
{"ErrorInfo":"","ErrorMessage":"Invalid json input: object ""->"TContact" has no property "firstname"","StatusCode":400}

The properties are already added, here is a screenshot showing it:

I've also tried creating contacts in lists directly (Replaced list ID with a dummy ID, before posting here)

$mj = new \Mailjet\Client($apikey, $apisecret, true, ["version" => "v3"]);
$listId = "123456";
$body = [
"Action" => "addforce",
"Contacts" => [
[
"Email" => "ashmit@xyz.com",
"IsExcludedFromCampaigns" => "false",
"firstname" => "Ashmit",
"lastname" => "Shetty",
"phone" => "123456",
"gender" => "M",
"birthday" => "22/09/1997",
"dexcom" => "Dummy",
"zugehoerigkeit" => "Dummy"
]
]
];
$response = $mj->post(Resources::$ContactslistManagemanycontacts, ["id" => $listId, "body" => $body]);
$response->success() && var_dump($response->getData());

But this gives me the Error, "Email is not valid" on the website..
I know I am probably doing something incorrectly but I am genuinely not sure what it is, any guidance on this is really appreciated. Thank you.

Edit: Would just like to mention if it is useful, I have tried creating a contact directly from the website without API and that works fine.