Does adh promote dehydration?

No, it doesn't. It actually promotes water retention inside the body.

Before your urethra, millions of tiny tubules are found inside your kidneys which are susceptible to the action of ADH.

When ADH acts upon them, it increases the permeability of their walls (the walls of the tubules). Hence more water can be drawn out of them and held inside the kidneys, not drained into the urethra. Then this water enters into the blood vessels and hence retained inside the body.

This leads to the excretion of a more concentrated, strong colored urine because it will lack water. The lack of water will mean that the volume of the urine will also be reduced, hence the initials ADH meaning anti-diuretic hormone (Diuretic = something that causes urination, anti-diuretic = something that prevents urination).