MDF is darker due to the binder used to hold the wood flour together.
You can use a coating, such as baking soda. I’ve not done it myself, but have seen the results from others.
Google ‘baking soda laser’
the other option is to slightly defocus - it tends to burn rather than ablate, leaving a charred residue.
EDIT: exactly the process I was talking about: Baltic Birch Plywood Engraving Settings?