For IPs used behind private URL access to HCP clusters, the IPs in the CIDR range of the HVN will be used.
For IPs used behind public URL access to HCP clusters in AWS, the IPs in the CIDR ranges of the cloud provider region your HVN is in will be used.
For AWS the list can be found here and for Azure the list can be found here.
Support for static IPs for HCP service endpoints is currently not available.
For additional questions or support, please open a Support ticket.