Dogs do not suck. If you are referring to a dog that seems to be sucking a penis completely inside his mouth, that is called the abuse of suckling reflex. This reflex sometimes remains in dogs from puppy age. It has nothing in common with sexual behavior or genital hygiene and taking advantage of this for your own pleasure is barbaric.
If a dog does not want to lick your dick, you accept that and move on. Forcing or baiting a dog to do that is either animal abuse or so fucking dishonest that even the mods on this forum remove such content.