Based on your post, I assume you are not following the code Program [1] path. In this case, you will need to have the client log an SR ticket to have your app allowlisted in their domain. Typically, they will go through a testing phase in their non-production/certification domain before putting the app in their production domain.
Once the app has been allowlisted in the appropriate domain(s), yes, you will update your discovery URL to
https://fhir-ehr.cerner.com/r4/tenant_key/ (or
sandboxcerner.com for non-production) where
tenant_key is the unique ID of the domain you are integrating with.
-Aaron (Oracle Cerner)