The HTML5 Report Viewer is a client-side widget served by a running Reporting REST service, which is dedicated to work with the viewer. It is the service that manages the report state and resources. For more details, please check this forum thread.
In the thread I explain how the viewer and the required REST service work.
Before proceeding with setting the viewer and the service, my recommendation is to configure the report use a data source component and a parameterized data retrieval method.
For example, you can use parameterized SQL queries and map the SQL parameters to report parameters. Where report parameters values can be passed though the viewer - How To: Pass values to report parameters. The viewer's reportSource.Parameters collection is mapped by key to the report's ReportParameters collection.
Details how to configure a data source component to use parameterized queries are available in the SqlDataSource Wizard help article.
I hope the above information is helpful.
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