If you do not know what to use, you likely don't know stainless soldering takes a different approach. Here is some help.
45% silver solder or better with white or brown flux to start but always point your flame at the copper allowing the ss to only get near the flame. once the solder melts onto the copper, move some heat onto the ss. And don't burn the stainless. It'll get black. Once you do, you will need to wash off your joint with cold water and do it again. Be sure your ss is clean to start with. Unlike sil-fas, you need to clean the copper & stainless with emory. Wash off harden flux with cold water and rag while the joint is still hot. Good luck.