For your requirement, we are recommending designing the report using Report Designer available in Embedded Reporting Tools Build Add On instead of using our demo server. You have to download Bold Reports Embedded Reporting Tools from our Bold Reports site using your Syncfusion registered account.
As of now, we don’t have the WebAPI datasource as default option with ReportDesigner. So, we request you to download the additional files from below link and replace it below mentioned installed location to work with WebAPI data source.
Location: $installed driver$\Program Files (x86)\ Bold Reports\Embedded Reporting Tools\Utilities\Report Designer