The following is a PHP example using CURL.
Activating a license key with a Device ID that is already in the database won’t decrease the remaining activations number.

Example License Key activation 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:
$post_url = '';
$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' => 'activate',
    // 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'           => ''
//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_SSL_VERIFYPEER, false);
// Execute post
$result = curl_exec($ch);
// Display the request result
echo $result;

Activation Successful Response

    "result" : "success",
    "code"   : "300",
    "message": "License key activated"

Maximum Number of Activation Reached

    "result" : "error",
    "code"   : "350",
    "message": "Maximum number of activation reached"

Expired License Key

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

Invalid Device ID

    "result" : "error",
    "code"   : "750",
    "message": "Device ID required, this license keys was activated with a device ID, a device ID is required to activate it again"

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"