The HTML5 Viewer is HTML/CSS/JS based widget, where you can update any of its templates. The updated template files will have to be merged manually on upgrade, and the files will have to be linked explicitly by the page - Providing templates.
An easier approach can be to create external custom UI that passes values to report parameters through the viewer's reportSource e.g. How To: Pass Values to Report Parameters.
I hope this helps.
Regards,
Stef
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