This feature is only available when sending a text to all active contacts or a group. To test this, add yourself to a group and send to just that group. This does not work with previews or single text messages.
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Simply use the (FIRSTNAME) tag in your messages. For example 

Hi, (FIRSTNAME)! Stop in today to get a free soda when you buy a slice of pizza. 

Also, you can use the Optional1, Optional2 and Optional3 fields with these tags: (OPTIONAL1), (OPTIONAL2), (OPTIONAL3).