Anti-dessicants of any type generally have very poor standing by most horticulturists. They may be marginally effective in protecting broadleaved evergreens against winter wind damage but they ave minimal beneficial impact in drought situations.

Transpiration or the loss of water via the leaf stomata is essential to the growth of the plant and its ability to photosynthsize. Anything that impedes that is going to be counterproductive. It is how plants "sweat" or cool themselves with high heat.

You would be better served by long and slow watering at the root zone. What is your watering regime like now?

The weather has been in the high 90s to low 100s. A young lilac (one gallon container) I planted this spring seems to be needing water. I water it every day, sometimes twice. I planted it in native soil, put mulch around it. Yesterday, I added some shade cloth to protect from the hot afternoon sun.

The shade cloth will help.

Watering with a hose. Watering until the basin is full and not draining. Yesterday, I aimed the water at the middle of the plant hoping to get water into the root ball. I also water around the edges to encourage roots to spread out seeking water.

Try just leaving the hose on a slow trickle for an extended period of time, moving it around the plant so the entire area is hydrated. And have you checked the rootball directly to see if it is properly moist? Often these will dry out before or just after planting and because of a different soil texture, are unable to rewet properly. So while you may be watering the soil surround the roots, the roots themselves are not getting enough.

And yes, the shade cloth will help. Much better than spraying the foliage with some foreign substance :-)