I don't use my surname on any Government ID ,but it is mandatory to fill surname while creating Certification Profile for AZ900 certification as then only you can continue creating your profile.
and it is also mandatory while appearing for Exam your Name should get match with that your Govt. ID,
then only you will be allow to appear for AZ900 exam.
Please let me know the work around , as of now I have filled my Complete name (First Name and Last Name) for Certification Profile.