From the conversations screen, choose the conversation you want to mention someone in. Click the square icon at the bottom left of the screen to enter expanded comment mode. In expanded comment mode, you will have a larger window to view your typed response and options for @mentioning, marking the comment urgent, and/or adding a picture to the conversation.
Click the exclamation point in a triangle icon to mark the comment urgent. The icon will turn orange to indicate that the comment will be highlighted in light red when posted.