Sounds like you're using tap water for a SW tank, since you ask if once the dechlorinator is in you can add salt - correct me if I'm mistaken.
The short answer is yes, it removes chlorine right away, but the long answer is that it's best not to use tap water for a SW tank - tap water has nitrates, phosphates, and heavy metals that you don't want to add to the tank. RO or distilled water is a better choice IMHO.