Regarding Certification

Can I submit my capstone project in my final project for certification test

first upload this on then copy this link and paste on the exam section

Yes, the Capstone project can be submitted as the final project for certification test, but it should be uploaded on first in your personal account. This link can then be used in the exam section.
However, it is highly recommended to create new projects using the Bolt IoT module as it will help in widening your concepts and help you to enhance your knowledge hereby using your creativity. So, surely try to build a project or two and upload them on because hey, the certificate is just a formality but, the actual knowledge lies within your mind.