We have SSO login setup via Salesforce for our platform, using which any authorised Salesforce user can access our service. In order to achieve this, we send an AuthnRequest to the Http-Redirect url that we received when in the metadata of Salesforce as ID provider.
The problem comes when a Salesforce partner user needs to access our service and tries to login via Salesforce. These users are directed to the main Salesforce login portal instead of the partner login portal and on that portal, their login credentials will not work.
If these users login to the partner portal first and then try to access our service then things work fine, since the ID provider in the two cases will be the same. Problem comes, when they directly try to access our service.
Is there any way to enable login for these users via Salesforce, perhaps by altering the AuthnRequest such that these users are directed to the partner login portal and not the main Salesforce login portal?