Hi @Dzongov-Sunwell,
You can add expressions via the expression editor. This way it will recognize the add function as well.

However, I don't think we can use this expression and the Get item action because you don't know the ID of the correct Annual Leave Balance list item yet.
In order to find the correct annual leave balance list item we need to match it to the leave request. My guess is that in both lists you have a person type column called employee, correct?
If so, I can create another example for you with a different approach.