Hi,
I don't think currently there is any parameter you can pass in URL to hide the filters by default. What I was told to this question in one of the webinars is that if we save the dashboard with filters hidden or shown, then when the dashboard is opened next time the filters will be hidden or shown respectively. So, as per that statement, if you save the dashboard with filters collapsed / hidden, when you call the dashboard via URL, they should remain that way.
Regards
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Kiran Passumarthi
www.linkedin.com/in/kiranpassumarthi------------------------------