You can always add you (un)availability in L1NDA. But there's also an option to check in on unfulfilled shifts. This means you're able to communicate your availability to your planner for specific shifts.
In the schedule you'll see red dots behind the unfulfilled shifts.
Click on the red dot if you want to work the shift.
The red dot changes in a green dot and your availability is added.
It is now up to your planner to be scheduled.