OIDC supports Initiating Login from a Third Party: https://openid.net/specs/openid-connect-core-1_0.html#ThirdP...

Unlike SAML's "take this assertion" IdP-initiated flow, OIDC went for a "start an authentication with this IdP, for this user, and send them back here". Much, much safer.

The "OIDC way" would be an endpoint on the service provider ("relying party" in the spec) that immediately redirects into authentication. The spec does have a section describing this.

https://openid.net/specs/openid-connect-core-1_0.html#ThirdP...

IDP-initiated flows are less secure, as they cannot prevent unsolicited logins. Last time I checked Google went as far as to block this flow in their Firebase Auth product.

https://www.identityserver.com/articles/the-dangers-of-saml-...