Lots of other replies before I saw this but I'll offer an answer anyway.

There is a technique called wire whipping. Look up whipping knot on Wikipedia. I whip the ends of the wire with 26 gauge bare copper wire (various places to get that) then squeeze the whipped end in a vise until it will fit in the slot; IMPORTANT: Then I put flux on the whipped, flattened end and tin it so solder flows into the wire and around the copper wire used for the whipping knot. Put a little flux in the slot of the ESC post, slide the flattened, tinned end into the slot, more flux on that, put a little solder on the iron tip, touch it to the tinned flattened wire until the solder flows into the solder post slot. Then it looks like this:



Something worth mentioning is that I first started soldering about 50 years ago (building slot car chassis from brass strips and piano wire). You don't learn to do this well by doing 5 solder joints.

Also important: I use only 63:37 Eutectic solder.