Will the screw be gripping into the MDF panel or into the Ikea panel (i.e will the head of the screw be seen on the MDF or inside the wardrobe?
The Ikea wardrobe is chipboard and is hard to get a good fixing into. MDF will grip much better.
If screwing into the MDF, try to clamp the two surfaces together along any exposed edges before you drive the screw in.
For really strong, as long as the heads won't be seen, use inter screws (these are the double-headed things you get with an Ikea wardrobe to join one unit to another).