Login into IDP without visiting IDP login page

Our existing application(app) handles authentication through devise.
An Identity Provider(IDP) is set up with devise and saml_idp.

Is this possible through programming?
A user logs into the application. During login process, a second login process
starts logining into an IDP and returns SAML.
I understand the application should use IDP for authentication; however, changing the application’s
login page route to IDP login is not an option.

Does Okta api have a post method with params? Is it possible to login into the
IDP without visiting the IDP login page?

Could you use instead OAuth/OIDC with prompt=none parameter?