Instead of using a custom parameter to filter the list, I recommend using the built in filter functionality. You should be able to add your EventID field into the GridFieldFilterHeader gridfield component.
But not quite what I’m looking for.
I think I need to give a little more context.
I use SilverStripe to manage events and associated shifts.
For ease of use I created a view with calendar sheets in the admin.
What I would like to achieve is that a user can click on an event in the calendar sheet, targeting the list view of that event’s assigned work shifts.
It boils down to the question of whether it is possible, and if so, how parameters can be inserted or pre-assigned into the gridfield or search form. Preferably by just using a URL with GET parameters.
I don’t know if that would be doable with CMS 4.11 - but when 4.12 is released later this year it has some improvements to gridfield navigation (namely the state of the gridfield is stored in and rehydrated from get variables) which would allow you to do this.