You're working with PHP, so your code should look like this instead... i.e. the POST body is a PHP array:
$response = Http::post('https://api.shoonya.com/NorenWClientTP/QuickAuth', [
"jData" => [
"apkversion" => "1.0.0",
"uid" => "dsfee3",
"pwd" => "deeddee",
"factor2" => "680208",
"vc" => "eefsdf",
"appkey" => "wefdsesdfd334",
"imei" => "abc1234",
"source" => "API"
]
]);
It is send as application/json by default