The best person to ask is the one you are buying from. There is no way for anybody here to know exactly what species of sponge you are getting so they can only suggest how to handle it and it would be more of a guess than knowledge.
That said, if it were me, I would never dip a sponge in Coral Rx or any other solution. I'd prepare a small bucket with your saltwater. Then open the bag with your new sponge and place the entire bag in the bucket of saltwater.Carefully get the sponge out of the bag so it is free in the bucket. Give it a careful examination and maybe even take photos if there is any kind of guarantee from the seller. Then take a container like a drinking glass or a new bag and put it in the bucket with the sponge and get the sponge inside. Now you can take out the glass with the sponge still fully in the water and move it into your DT. This will minimize the original water from the seller getting into your tank, still protect the sponge from being exposed to the air in case that would damage or kill it and give you a chance to give it a serious examination.
If the seller knows the sponge well, they may be able to tell you it is tolerant of exposure to the air and if it can be dipped. But anybody else would be guessing unless they bought the same sponge from the same seller. I'd go to that extreme protocol for getting your new sponge in your tank even though I know of a red branching tidal sponge in the Gulf of Mexico that can sit on the beach for an hour or two and still survive. There are just too many species of sponges (literally thousands) with different characteristics and many that can be nearly impossible to tell apart without a microscopic examination.