FHIR works on AWS server not allowing to keep customized id as primary key

We are working for FHIR(Fast Healthcare Interoperability Resources).

We have followed “FHIR works on AWS” and deployed the CloudFormation template given by AWS in our AWS environment.Following is the template that we have deployed

https://docs.aws.amazon.com/solutions/latest/fhir-works-on-aws/aws-cloudformation-template.html

Requirement : we want to maintain client specific/customized ids as primary key in the server.

Problem : server not allowing us to override or mainain client specific (customized ) ids as primary key .Infact , in the runtime, it is generating its own ids and ignoring the id given by us.