What is the expected format to pass request body while calling https://api.intercom.io/contacts

I am trying to fetch 150  contacts per page while calling  https://api.intercom.io/contacts.

but on trying to do so , i am getting the following error 

{"type":"error.list","request_id":"0069rm2ubh0lgdfr7kg0","errors":[{"code":"invalid_document","message":"either_email_or_user_id_present validation failed"}]}

attaching the REST API INTEGRTION code below: ( implemented using Salesforce Apex)

String endpoint=’ https://api.intercom.io/contacts’

HttpRequest request = new HttpRequest();
 request.setEndpoint(endpoint);
 request.setMethod('GET');
 request.setHeader('Authorization', 'Bearer ' + INTERCOM_API_TOKEN);
 request.setHeader('Accept', 'application/json');
 request.setHeader('Content-Type','application/Json');
      
 Integer perPage = 150; 
 String json= '{'+            
            ' "per_page":'+perPage + 
  '}'; 
  system.debug(json);

 request.setBody(json);

Could you kindly share your thoughts on how to handle this error . Any input is highly appreciated .

Thank you