The abilities of Jink and Markerlights apply at different phase of the combat and neither need to be declared until they are about to be utilized.
Jink: A model with this special rule that has moved in its Movement
phase gains a 5+ cover save until the start of its next Movement phase.
When a unit with any models with the jink special rule is selected as
a target for a shooting attack, you may declare that it will jink. the
decision must be made before any of the to hit rolls have been made.
If the unit jinks all models in the unit with this special rule gain a
4+ cover save until the start of their next movement phase, but they
can only fire snap shots until the end of their next turn.
In short, for models with Jink, this special rule automatically applies provided the model has moved in its movement phase and does not need to be declared until the Tau attack which applies the Markerlight.
Instead, each time a unit suffers a hit with this special rule, place
a markerlight counter next to it (no saves can be taken against these
Immediately before a unit from Codex: Tau Empire shoots at a target
that has one or more markerlight counters, it can declare it is using
one or more of the markerlight abilities listed below.
So in your situation in which the Markerlights are already on the jetbikes, Jink has already been rolled for (or should have been). The order goes like this:
- Tau with special rule 'target acquired' attacks jetbikes
- Jetbikes-controller declares Jink rule prior to to-hit rolls and takes cover saves in response to successful hits
- A successful hit applies markerlights to the unit
- The Tau player can declare Scour/Seeker to add Ignore Cover
- The Jetbikes have no cover during this shooting phase.
If Pinpoint is declared, then no Ignore Cover special rule is added. When the Tau player attacks, each Jetbike unit may claim Jink (again provided they moved in the movement phase) and take their 5+ or 4+ cover save.