You do not buy "virtual user minutes (VUM)" before doing the testing, you set up an account that allows Microsoft to bill you for the VUM that you have used. Just before a load test starts the system will inform you of how
many VUM the test will consume and, at that point, you could abandon (ie cancel) the test before it starts running.
There are some free minutes. Beyond that, Microsoft say "If you need to pay for more amounts of these services, you must first set up billing for your Team Services account. You can then pay for these services in the Azure classic portal or the
Azure portal. Learn more about pricing for these services.". See
https://www.visualstudio.com/docs/setup-admin/team-services/get-more-build-or-load-testing-vs
There is a FAQ, see
https://www.visualstudio.com/load-testing-faq-vs
Having created the necessary account for billing, you need to make sure that load testing is allowed to use more than the free VUM. Log in to your "manage.windowsazure.com" account (I am not sure of the correct URL to access the account, but
you can start at
https://beta.visualstudio.com/team-services/cloud-load-testing/ and then sign in). You should then find a page similar to the image below, and select "Paid" for the load testing.

Regards
Adrian