I like to put it in a pair of visegrips set to just barely gripping it standing on end. Vise grips laying down, Take the tip of the iron and stick it in the hole, Hold it there for 15 seconds. It will then be hot enough to take the solder. Silver solder, no flux core.
Just feed it in. till it is about full. concave shape of solder
Tin the wire prior. Heat the solder on the wire and then stick it in the hole pointing the wire down.
I have my iron set on hottest setting and not sure what you are using.

Hope that helps, oh and put the piece of heat shrink tubing on the wire back a bit first!

Jeff