Hi Rohit,
Have you tried scheduling your AZ-900 to see if the discount is automatically applied? You need to be signed in using same email/account that you used to sign up for Virtual Training Day.
If you get all the way to the final page where it shows price and asks for payment without showing discount, please use link below to create Certification support request:
https://aka.ms/credentialssupport
Creating support request screen should appear similar to below. You may need to select different options depending on your needs:
Please click Accept Answer if the above was helpful.
Thanks.
-TP