I would automatically enable the filter "Everybody's calendars" by calling an act_window action mode calendar.

Something like this:

<pre>

        return {
            "type": "ir.actions.act_window",
            "name": self.name,
            "res_model": "calendar.event",
            "view_mode": "calendar,tree,form",
            "domain": domain,
            "context": {
                'default_filter_partner_ids_all': True,
            },
        }

</pre>

What is the simplest way ?

Thanks