Continuous Footnote mark

I don't think it can be achieved automatically - I don't see any option for that in InDesign's Footnote options.

But it can be done manually - Footnote Reference mark is just a marker - and unlike auto-numbered lists - is "movable" - you can insert text before it:

I've manually copied "original" Refeerence marker from the top frame and split the footnote's contents - into two lines - and pasted this original marker and added text before.

And here is text after adding a new Footnote:

The duplicated marker is updated automatically by InDesign.

But as you can see - it will require moving marker and (cont) text again.