How to handle authentication for integrating app in Box Custom web app integration ?

Hi,

I would like to integrate a third party app in Box using Custom Web Integration.

For Box authentication, I'm getting code at the redirect URL & I can make use of that code to call Box APIs.

Here, I couldn't know how can I handle the authentication for my app, which I'm integrating here.
My integration will be like 'Sign with Docusign' integration. When I'm clicking this option from Integrations menu, I' redirected to accounts.docusign.com for signing in to docusign. Where do I handle this authentication ?