You are better off mixing it into the soil a couple months ahead of time instead of at time of planting, but the quantity depends on how truly composted it is. If it looks and smells like poo still, it should not be mixed into the hole and the amount spread on top depends on the watering requirement... if it were in a small pot and hot weather so frequent watering is needed, less manure should be a top dressing and vice versa.
If instead it is a raised bed, simply spread it out over a little larger area instead of right above the root ball. In other words you have space between each seedling where it can be spread and tilled in ahead of time. Manure is fairly low in carbon so along with the manure, composted or not, there should be ample brown material as a carbon source.
Untreated wood sawdust, peat moss, brown leaves/stems/bark, pine needles, brown grasses and weeds (minus any seeds), etc. are best mixed into the soil a month, or better two, before transplant, along with the manure, and kept damp below the soil surface.
Adding more later during the season tends to attract insects laying eggs in the soil that then infest the plants, fungal, and bacterial growth, though if it is completely composted (looks like dirt) and the soil surface is always allowed to dry out after watering (and no excessive rain) it will be less of an issue. Even so, ideally you don't want a layer on top of the soil, rather a mulch of some sort that dries faster and retards weed growth.
