The following is a PHP example using CURL.

Example License Key deactivation API Request

// The post url is your WordPress website URL where the plugin is installed
// If the your WordPress installation is in a sub-folder the URL to that sub-folder
// should be used instead. Example: https://domain.ltd/my-sub-folder
$post_url = 'https://domain.ltd/';
 
 
$parameters = array(
    // The API command
    // The fslm_v2_api_request parameter takes one of the following values
    // verify, activate, deactivate, details, extra_data
    'fslm_v2_api_request' => 'deactivate',
 
    // Your API Key
    // You can set your API key in the page
    // License Manager > Settings > API
    'fslm_api_key'        => '0A9Q5OXT13in3LGjM9F3W',
 
    // The License Key
    'license_key'         => 'FFFF-FFFF-FFFF-FFFF',
 
    // The device ID
    // The Device ID is optional, but if it is used and the license keys was activated with it
    // it becomes required, a license key activated with a Device ID can't be deactivated
    // without it and can't be activated again without it.
    // The device ID can be anything you want, its role is to identify the "Device",
    //  "Machine" or "Domain" where the license was activated.
    'device_id'           => 'userdomain.ltd'
);
 
 
//url-ify the data for the POST
$fields_string = "";
foreach($parameters as $key=>$value) {
    $fields_string .= $key.'='.$value.'&';
}
rtrim($fields_string, '&');
 
// Open connection
$ch = curl_init();
 
// Set the url, number of POST vars, POST data
curl_setopt($ch, CURLOPT_URL, $post_url);
curl_setopt($ch, CURLOPT_POST, count($parameters));
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields_string);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_USERAGENT, $_SERVER['HTTP_USER_AGENT']);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
 
// Execute post
$result = curl_exec($ch);
curl_close($ch);
 
// Display the request result
echo $result;

Deactivation Successful Response

{
    "result" : "success",
    "code"   : "400",
    "message": "License key deactivated"
}

License Key Already Inactive

{
    "result" : "success",
    "code"   : "450",
    "message": "License key already inactive"
}

Expired License Key

{
    "result" : "error",
    "code"   : "550",
    "message": "Expired license key"
}

Invalid Device ID

{
    "result" : "error",
    "code"   : "700",
    "message": "Device ID required, this license keys was activated with a device ID, a device ID is required to deactivate it"
}

Invalid License Key

{
    "result" : "error",
    "code"   : "100",
    "message": "Invalid license key"
}

Invalid Parameters

{
    "result" : "error",
    "code"   : "600",
    "message": "Invalid parameters"
} 

Invalid API Key

{
    "result" : "error",
    "code"   : "200",
    "message": "Invalid API key"
}

An Error Has Occurred Please Retry

{
    "result" : "error",
    "code"   : "000",
    "message": "An error has occurred please retry"
}