I want to schedule certification exam for Nintex workflow and Forms O365. Please share link from where i can book/schedule a slot ?

https://learning.nintex.com/

Afaik there is no need to schedule your exam. You can just register on the site and start the exam whenever you want. This exam is free of charge (if they haven't change this). Since you can do the exam online you can do it from anywhere in the world as long as you have a stable internet connection.

More information on the exams can also be found here: https://www.nintex.com/blog/can-earning-a-nintex-certificate-can-benefit-you/ 

Cheers

Philipp