You will need to know your OpenAthens domain name. This is usually the same as the scope registered against your domain organisaiton organisation as seen on the organisation summary.
E.g. if your OpenAthens domain is institution.ac.uk, your SAML 2 metadata address will be:
If you want to view this in your browser you may need to add a ?browser paramater parameter to the end of the link, e.g. https://login.openathens.net/saml/2/metadata-idp/institution.ac.uk/c/oafed?browser.
Manually specifying connection settings
This will be the x509 certificate in the metadata, topped and tailed as follows. This is sometimes called PEM format.