I do not have the subscription, so I do not know about the particulars in detail. I would assume that you get a voucher code as part of your package. With such a voucher code, you can book the exam for free; at least that is how other discounted or free GCP exams can be booked.
Do you have your voucher code? If not, look for it in the welcome email or write to the contact address surely contained therein. If you cannot find a contact address, you could use
Contact Us - Cloud Certification Help
Once you have your voucher code, you can create an account and log in to
Login | CertMetrics
From certmetrics, you can schedule your exam. This will take you to the webaccessor page where you can use your voucher code.
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