Disclosed is a method of operating a terminal on which an application for an instant messaging service is installed, the method that displays a first user interface (UI) for switching an input mode in a chatting interface of a chatroom in which a user of the application is participating, based on whether the user is assigned an authority to emphasize a message in the chatroom, switches the input mode from a first mode which is a normal mode to a second mode for emphasizing a message, in response to a user input with respect to the first UI, emphasizes a message input in the second mode for a predetermined period in a predetermined manner, and cancels the emphasizing of the message input in the second mode, when the predetermined time elapses.