You need to first find the salinity of your water by using a hydrometer or refractometer. Then look at the instructions of the salt bucket/bag that you got. It should tell you how much to add to get the salinity you want. For example, I use Red Sea coral pro salt so to get a salinity of:
1.021 I need 33.4 g/l also known as (1.4 lb/5 us gal
1.023 36 g/l (1.5 lb/5 gal
1.025 38.2g/l (1.6 lb/5 us gal
Different brands will have different amounts for the proper salinity.
I convert lbs to grams then grams to ounces. Involves some calculating!
You should allow the salt to mix and dissolve for 24 hours before adding it to the tank as undissolved salt can harm fish/inverts. Since you have a small amount of water, you'll have less room for error meaning the salinity should be very close to what it is already or it might drastically alter. Good luck and test the salinity before you put it in the tank.