how to authenticate user with different page and then redirect back to vbcds

Summary:

Hi Experts

We are building VBCS extension for fusion for one of the UAE based customer. We have a requirement to authenticate the user with UAE PaaS.

problem here is that clicking on login button has to route user to use pass website i.e. htts://uaepass/userlogin/sample/dummy and after user provides all details it will redirect back to the page what we want (we can provide the link for the vbcs page) but it will have to query params in the URL code which we need to get from the redirect url https://redirecthost/?code=bndfjdfjdk