I know it is possible to select a position on bar or a note value in advanced filters. The note i want to select is the first beat in bar. How could you select it without interrupting the whole score?
I'd imagine you're looking for something like this, yes?
This isn't entirely right, in that it is treating the pickup like a full bar, so I would clear those notes with Ctl-click. I also work with an older version of Sib (4), so your mileage may vary (although I don't imagine the filtering options have changed very much).
What I did here was to work with advanced filtering for "Notes and Chords", clear the "Any" checkbox under "Position in bar", choose a semibreve value, and check the "Plus multiples" checkbox.