Multipass is the way to go here, what's not working for you? The instructions for using Multipass including an example implementation are available in the docs here.
Thank you very much for the reply. So actually the problem here is that we want to authenticate the customers which already exist in shopify store based on email and password for our external website. We found that Multipass provides the way to authenticate based on email only. So is there any way to authenticate the customer by using email and password both. (on external websites).
Awaiting your reply. Many Thanks.