Hi,
You may apply your voucher to any one of the four eligible exams (AI-102, AI-900, DP-600, DP-100) regardless of which challenge you completed. When scheduling exam, you need to enter your voucher code on the Enter payment and billing page, as shown below:
After entering your voucher code and clicking Apply, you should see 0 for Estimated Total Due, similar to below:
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP