How can i have access to the Sandbox of my shopify account

[saiyugesh]

 We have some clients who actually uses the paid version of the shopify. We want to integrate some on premise application with shopify through the Shopify API. My question is how can have access to the sandbox account of my clients shopify account? Do they have to contact the shopify from their end or we can do it on our own.

Thanks

Sai Yugesh