Answer:
Scoop about 100ml of the water where you found it into a large plastic container and gently place it inside.
After doing so, poke holes on the lid of the container with something that has a sharp point and close the lid, leaving it by itself in a dark corner.
Meanwhile, fill a giant container/bucket with tap water and leave for one day for the chloride to evaporate.
If you bear for any fish to possibly die, place small fishes like guppies with the crayfish as food. You can also feed your crayfish elodea, a water plant, bloodworm, small snails or perhaps tiny insects such as beetles. Crayfish also eat dead water plants. Remember to feed it as much calcium as possible!
If possible, buy a tank with ample space for the crayfish to swim around. Include some gravel on the base of the tank or maybe some water plants or rocks.