Use silver solder and a hot iron.
Sand the end of the rod to roughen up the surface.
Test fit.
Apply flux to the rod before soldering (use flux made for silver solder)
Method 1: Get the adaptor hot and fill the opening about half way with solder (IF the adaptor has a hole in the side follow method 2) keeping heat on the adaptor insert the rod into the adaptor (be very carefull to not allow solder to spill onto the threads)and keeping them together remove heat.
Method 2: same as 1 but instead of filling the adaptor you assemble the rod into the adaptor first then apply heat and allow the solder to wick in.
PS: IMO it would be easier to use a solder on clevis and if you insist on using a ball link put that on the threaded end.