For a service called mOTP, I need to send a parameter with name \'private\'. Here is my Parse cloud code.
var phnNum = request.params.phnNum;
var validationParam         
        First of all, You should pass a body message with the POST method.
The content-type should be 'application/x-www-form-urlencoded' according to the documentation.
Parse.Cloud.httpRequest({
    method: 'POST',
    url: 'https://motp.p.mashape.com/v1/otp/{APIKey}/{SessionId}',
    headers: {
        'X-Mashape-Key': 'YOUR-X-Mashape-Key',
        'Content-Type': 'application/x-www-form-urlencoded'
    },
    body: {'private': 'Your_mOTP_Private_Key'},
    success: function(httpResponse) {
        response.success(httpResponse.text);
    },
    error: function(httpResponse) {
        response.error("error: " + httpResponse.status);
    }
});