When should I apply Minoxidil on my scalp? The crown area and the hairline. After the shower? The hair must be wet or dry?

Use minoxidil as directed. The instructions are usually written on the box or on the back of the bottle.  Since it is a medication that needs to go on your scalp, it would make sense to use it when your scalp is dry.

I like telling my patients to take a hot shower and then dry their hair lightly, put the minoxidil on the SCALP, not the hair as much as possible. The foam gets all over the hair while the liquid can be placed on the scalp. Once a day treatment is ok, provided you leave it on overnight, then you can wash your hair in the morning.