Hi,
Welcome to Microsoft Q&A community forum!
This forum does not deal with certification related topics, you can refer to this link for support.
https://trainingsupport.microsoft.com/en-us
However, if you have a voucher, you must go to the exam page,
https://learn.microsoft.com/en-us/certifications/exams/az-900/
click on schedule with Pearson Vue and once you have followed
the procedure at the time of paying, you can enter the code of your vocuher, and the amount of the voucher due will be deducted from the final price
Hope this helps
Cheers
Luca