You have a 'portable' water softener
It uses regular table salt so that the salt can dissolve easily/quickly during regeneration cycle.
Other salt products will work.....but you will need to crush it to size of table salt for it to be as effective/dissolve as quickly.
WHY reinvent the wheel.......mfg. has already done the testing. Use the table salt. It is cheap/easy to store/easy to find-purchase and it works very well with your portable softener.