If you have a HOB filter toss a handful or 2 of rock salt in the morning, then another couple in the evening, and test until you get to the salinity or specific gravity you want to achieve.
Or if you have a sump toss it into a tray or right in the sump.
I do this if I get a case if ich, and test until the salinity tops 3ppt, usually takes 2 days with no ill effects so far.