There is no rule that you must have a hay rack. They are merely a convenient place to put hay. The main benefit of a hay rack is keeping the hay off the floor where it may get soiled by urine. However, that might not be an issue for you if your rabbit is well litter trained.
How do you use a hay rack for rabbits?
https://www.youtube.com/watch?v=1dMlxj-_IWc
Where should you put hay for rabbits?
Hay can be given to your pet in a variety of ways including in a hay rack attached to the side of the cage, in a box or basket within the cage or exercise area, or even placed in the litterbox. Rabbits often pass stool when they are eating and placing some hay in the litter box can help with litter box training.
How do I keep my rabbits litter box from smelling?
https://www.youtube.com/watch?v=DPMlRVUNZ00
Do bunnies need bedding in their litter box?
Rabbit bedding is a recommended requirement in a litter box but an optional one in a cage. Maze rug down and seagrass make excellent bedding for chewers while towels and mats are recommended for non-chewers. Many types of rabbit beddings are available, each with distinct advantages and disadvantages.
What is the best thing to put in a rabbit litter box?
CareFresh
How much hay should I put in my rabbits litter box?