Universal Containers (UC) plans to use a SAML-based third-party IdP serving both of the Salesforce...

The recommended way to configure the IdP for seamless access is to use IdP-initiated SSO that passes the SAML token upon Salesforce resource accessrequest. This means that the user logs in to the corporate portal first, and then clicks a link to access a Salesforce resource. The IdP sends a SAML response to Salesforce with the user’s identity and other attributes. Salesforce verifies the SAML response and logs in the user to the appropriate Salesforce org and community12. This way, the user does not have to log in again to Salesforce or enter any credentials3. References: 1: SAML SSO with Salesforce as the Service Provider 2: Set Up Single Sign-On for Your Internal Users Unit | Salesforce - Trailhead 3: What is IdP-Initiated Single Sign-On? – OneLogin