How do we apply an Identity Provider to an Application?

We have an application set up using local Okta users, and this is working great. Our application diverts to Okta, people log in, and then return to the application.

We have an Identity Provider set up pointing at our ADFS service, and so far this seems to have been set up correctly.

We are stuck at the point of integrating the two - how do we assign an Identity Provider to an Application, such that when you attempt to log into the Application it gives you the option to log in using the Identity Provider of our choice?

(The docs are very difficult to search for this, given that you can use SAML between an Application and Okta, and SAMP between Okta and an Identity Provider, I’m always finding the opposite of what I need)

https://help.okta.com/en/prod/Content/Topics/Security/Identity_Provider_Discovery.htm