In order to log in to a Portal for ArcGIS instance using a SAML-based Identity Provider, you will need to Register AGO-Assistant as an application in your Portal, to generate an AppID that can identify this app as an allowed client of the Portal. To do so, follow the instructions here, using Application as the Type of App and as the Redirect URI.

This login method is used for Portals with web-tier authentication, using username and password or client certificate (PKI) authentication methods, often referred to as “SSO” or Single Sign On). This authentication method is most often used for internal-facing Portals inside of an intranet. More info