How to Make user login automatically by using credentials from another source

How to Make user login automatically by using credentials from another source

Hey,

I have a SaaS based product, We have hundreds of users using our solutions. 

Here is scenario where I need help?
  1. I would like my users to create from tickets directly from my software page. 
  2. I would like them to check previous tickets logged by them.
  3. Important point is, I don't want them to create a new username and password for DESK, I would like to use my software User Credentials to allow them login and create/ check tickets.

How do i achieve above scenario?