How to make Lex replay the user AlphaNumberic Input.

I am busy building an IVR system. The user interating with the bot is asked for a reference number which is of type AMAZON.AlphaNumeric. When it is time to confirm the input, I wan to replay the reference number back to the user to confirm their input. What I want the bot to do is say out the reference number character by character NOT as a word. What Amazon Lex is doing is try to spell out a reference number like "AZkq239U". That's not what I want I want the bot to spell the reference number character by character e.g "A", then "Z", then "k" etc. How do I do this?