@RestResource(urlMapping='/jtelACD/genericQuery')
global with sharing class JTELACD_GenericQuery {
    // Class to decode the parameters
    public class GenericQueryParams {
        public String Param1;
        public String Param2;
        public String Param3;
        public String Param4;
        public String Query;
    }
    
    @HttpPost
    global static String doPost() {
        RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;
        
        // Deserialize the JSON (not used in this example - just for illustration purposes)
        GenericQueryParams genericQueryParams= (GenericQueryParams) System.JSON.deserialize( req.requestBody.toString(), GenericQueryParams.class );
 
        // Find next task to schedule in jtel system.
        // Note - the exact logic of this will be dependent on the Salesforce installation - this is JUST AN EXAMPLE.
        List<Task> tasks = [SELECT FIELDS(STANDARD),
                                   TYPEOF Who
                                         WHEN Lead THEN FirstName, LastName, Email
                                         WHEN Contact THEN FirstName, LastName, Email
                                   END
                          FROM Task
                          WHERE Status = 'Inbound Email'
                          WITH USER_MODE
                          LIMIT 1];
                          
        if( tasks.size() >= 0 ) {                    
            return JSON.serialize( tasks[0] );
        }
        res.statusCode = 404;
        return null;
    }
}

curl --silent -i -X POST -d '{ "Param1" : "Example Param 1", "Param2" : "Example Param 2", "Param3" : "Example Param 3", "Param4" : "Example Param 4", "Query" : "Example Query" }' --header "Content-Type: application/json" --header "Authorization: Bearer <OAUTH_TOKEN>" --header "Connection: Close" "https://<SALES_FORCE_INSTANCE_URL>/services/apexrest/jtelACD/genericQuery"