Aquarium salt should not be used for Bettas unless there is a specific reason to add it. Bettas are sensitive to salt, and constant exposure to it can harm their kidneys.
If you rescue a betta, try keeping it in warm, clean water. Many issues are 'cured' just by doing this.
If you get a fish with specific problems, tell us what they are. Then someone can point you towards the proper treatment....
But I would try just clean, warm water first. I would not add aquarium salt unless there was a very specific issue that warranted adding it.