You should not pour the concrete slab before building your pole barn. After placing the posts and placing the skirt around the perimeter of the posts, you will have a way to pour the concrete base. My personal preference has always been to pour water after the building is built, since it allows working outside of inclement weather. If it's cold, the building can be heated, if it's hot, the concrete doesn't cure too quickly, there are no unexpected storms to deal with, etc.
Welcome to Ask the Pole Barn Guru, where you can ask questions about construction topics, with the answers published on Mondays and the 26th on Saturdays. If you decide to pour the concrete after the building has been built, there is a potential risk of damage to the building. In short, the height of the building and the size of the swing door will help determine whether the pad should be poured before or after the construction of the building.