How to define a parameter that will be used as an IN argument inside a function?
Use an array and the ANY operator:
CREATE FUNCTION public.get_employees_id(P_IN_SALARIES int[], P_IN_DEPARTMENTS_CODES text[])
...
    select STRING_AGG(employee.id || '', ',') into v_employees_ids
    from employee 
    inner join departament on department.id = employee.departament_id
    where employee.salary = any (P_IN_SALARIES)
    and department.code = any (P_IN_DEPARTMENTS_CODES);
...

When calling the function you can use any array notation you like, I prefer array[...]:
select get_employees_id(array[100,200], array['code1', 'code2'])