I don't really know. Having looked at the part on the Pieces Fulvia website, I can see what it does, but no clue as to the correct method of attaching a wire to it. I don't want to remove the steering boss if I don't have to, so for now I've put a very small crimped spade terminal on the end of the wire, folded it over and pushed it in the hole to contact the spring. It doesn't come out easily and the horn works fine.
I would prefer to do a proper job if only I knew what that was!