This question already has an answer here:
- When should I use “a” vs “an”? 10 answers
Which one is better and why? 1) Are you an Ozibox user? 2) Are you a Ozibox user?
Ozibox is the name of a web application
In this case, use #1. The rule goes that one should use "an" before vowels unless the word begins with a consonant sound or the Y sound. In your example, "Ozibox" begins with the "o" vowel sound. See here for more info.