integrate authentication of legacy desktop app with new saas web app

have a legacy .NET desktop app. separate instances of code and db for each customer. user credentials in each db. .NET 4.5. We are starting a new saas web app. We want to allow SSO with UI and authenticated apis for a unified experience between the legacy and saas features. I need sample code on both sides which enables this. This is pure POC. No need for anything from existing code. Assume a user table in each customer db, with userid, and pw. You can propose whatever you want on the saas side. We are using the single code/ multi db saas model; the saas has access to all existing customer db's. If interested, you should describe previous deep experience with federated authorization, tools such as oauth, heavy saas implementation experience, and previous experiences like this. You don't need to flesh out the whole detailed approach to be chosen, but those indicating the simplest, cleanest approach are most likely to be chosen. Regarding the Dollar amount proposed, I am just throwing out some number here to get started, you can express need for specific hours. Thanks Deliverables Sample code where the user logs in as normal to the desktop app, and the user is also logged in to the saas. for the POC create some dummy table in the customer db, name it POC. It can have one row and one column. the value will be different for each customer. After login, the user then clicks a button which calls a saas api. The api requires authentication. The api will return the POC table column value. If this works, this proves the user has been routed by the saas to the correct customer db.