Niagara particles spawn forward calculation

How to make Niagara particles spawned as if it was already spawning ten seconds right from the start? Remember seeing a video about that, just cant remember where.

Tried to make this with niagara timeline and that doesn’t do anything.
It wont let me move the green thing left or right - it’s kind of greyed out.
Tried with delay , same thing. Did not find any solution in the documentation either.