@Ricardogolez I'm not sure what Shell Plugin Command you mean but..
If your going to regularly use this, I'd suggest a Laravel CURL package for easier readability, like this https://github.com/ixudra/curl
Otherwise, learning to CURL in PHP http://www.hackingwithphp.com/15/10/2/your-first-curl-scripts
Assuming your POSTing to the URL, it'll be something like this.. (untested)
$data = [
'color' => 'rainbow',
'message' => 'NEED HELP',
'notify' => false,
'message_format' => 'text'
];
$headers = [
'Accept: application/json',
'Content-Type: application/json'
];
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://url.com");
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$results = curl_exec($ch);
curl_close($ch);
echo $results;