reference from below link:

http://aspsolutionkirit.blogspot.in/2014/03/send-text-or-bulk-sms-in-aspnet-using-c.html

followed below steps

1. Register at Site2SMS and get Username and Password.
2. Register at Mashape.com and generate key.
3. Download the .DLL file from ASPSnippets.SmsAPI.dll click here and add it in your project bin folder

registered with site2sms
and register at mashape.com and used generated key …but error message show Invaild mashape key