Make client API in .net

Thank you so much for the help!!!
Attached URL is very helpful for me , but i am little bit confused here.One of our client use OAM(oracle access manager) provider and they are using our Web Application .

They want to open our Web App without username password separately ,just their user sign-in first time with domain and our Web App should open also after clicking on button.

So i want to write middle-ware API in .net MVC that communicate with OAM provider (or okta) and our Web Application ,after authenticate successfully with OAM provider our Web Application should launch on the screen without asking any login credentials or social credentials.

So this is my problem.Please help me the best example which match with my scenario and that sample also fulfill my requirement.
Same problem i want to test with Okta provider,so its very helpful that share the code sample for SSO implementation.

Thanks,