What *exactly* are you trying to treat?
In addition to this threat, I see you also have another thread asking about combining Epsom salt with Furan 2. I replied already on *that other* thread. On both threads, you're asking about combining medications/treatments for both internal and external issues.
* Aquarium salt is for external issues. It can contribute to swelling and fluid retention in freshwater fish (such as bettas).
* Metronidazole is generally used to treat internal parasites or anaerobic bacterial infections. (However, internal infections often cause bloating. So if your fish has these, you may want to rethink using Aquarium salt, as it can worsen the fluid retention issues.)
Is there a reason that you want to combine these? (Or, conversely, combine the Furan 2 and Epsom salt that you asked about on your other thread?)