We use the object to grow our quality of mind. We can use whatever object; the object is helping you to grow your awareness, wisdom and samadhi.
So, you can use any object, any of the six sense doors; not only one object is the Dhamma. Dhamma is everywhere; everything is Dhamma.
When we use awareness to lead, we’re growing the other good qualities. When we’re mindful, awareness, samadhi, effort, faith and wisdom are working together. Every time we’re mindful, we’re growing these five qualities of the mind.
When we practice mindfulness meditation, there are 4 foundations of mindfulness: Kaya, Vedana, Citta and Dhamma. These are objects. Object is already there; the important thing is that the meditating mind must be wholesome mind, skillful mind. The quality of the meditating mind must be good.
We should not practice with craving, aversion and delusion.