minnflyer has it right. using a big tip(clean) and a higher power soldering iron help(40watt).

hit the rod with a bit of sandpaper to rough it up and remove any oils or coatings, wipe clean.

Heat rod and touch solder to other side till it wicks on to the end of the rod(tinning the rod) only barely get any solder on it so the control will still slide on. Then do the rest like in minnflyers pic. I use fine solder wire 60/40 rosin core. Less solder is better usually.