How to whitelist an RDS endpoint

When you add the security group rule, choose Custom, then input the ID of the RDS's Security Group (sg-abcdefg)

https://docs.aws.amazon.com/vpc/latest/userguide/VPC_SecurityGroups.html#adding-security-group-rules

In this way, you are whitelisting all instance that use that particular Security Group sg-abcdefg (Including the RDS instance)

To avoid over whitelisting, I would also suggest creating a dedicated Security Group for the RDS instance.