As was mentioned before, shower tents will either be designed to be used with shower bags, or they will not be. If they are then, if you are lucky, the seller will provide you with the shower bag. If they do not provide you with one, then you should reach out to them to see what their weight limit is.
The tent needs to be able to support the weight of the water that is in the bag. Otherwise, the container may fall, or the tent may collapse down (which is the worst-case scenario).