Hi Farida,
It can take up to 5 business days after the event to receive the thank you email with voucher. If it doesn't arrive in your inbox by the 24th, please ask a question in Microsoft Credentials forum.
The email should have subject of Thanks for joining the Data Fundamentals training day. Scroll down to bottom where it says "Here is your unique voucher code:" and it will have your discount voucher code.
You need to enter this code when you get to the Enter payment and billing page during exam registration, as shown:
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP