Hello,
You can follow the instructions in the SAML2 docs: Configure IDPs, SPs, and CoTs :: AM 7.3.0
In a nutshell, you need to:
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Create a Hosted SP
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Obtain and import the metadata of the external IDP to create a remote IDP
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Create a circle of trust and add both entities to it
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Provide the metadata of your hosted AM SP to your external IDP
The assertion consumer URL is part of the metadata and can be accessed at https://openam.example.com:8443/openam/saml2/jsp/exportmetadata.jsp?entityid=myHostedProvider&realm=/mySubRealm (adapting the URL to your environment).