1. A method of transmitting data to a server by using an open application programming interface (API) including at least one field, the method comprising: acquiring basic data corresponding to a field from among the at least one field defined in the open API and extended data not corresponding to the field; appending the extended data to the basic data to generate a message; generating a Hypertext Transfer Protocol (HTTP) request message describing the message as a field value of the field; and transmitting the HTTP request message to the server, wherein the message comprises identification information indicating a position of the extended data in the message, wherein the open API is an API which is published by a service provider operating the server for providing a service related to the at least one field defined in the open API, wherein the basic data is processed by a first module and the extended data is processed by a second module, and wherein the message comprises identification information regarding the second module for processing the extended data.