How can I plug my homemade Yagi Antenna into a wifi USB dongle?
The solder-resist is absent from the marked area, suggesting that this connection point may have been used by the manufacturer to verify that this dongle meets a RF specification. It appears to be close to the antenna feed point.
That said, there's no way to tell what impedance the RF sees at this point, or how you could impedance-match to this point.

Andy has pointed out that a balun would be good where you attach the unbalanced coax to the balanced driven element. This could be a ferrite sleeve slipped over the coax. There are alternative balun approaches. Unbalanced drive can skew directivity.

A caveat: Any blog boasting easy-to-build 2.4 GHz directive antenna is suspect. Your antenna is a very narrow-band design - measurement tolerances are critical, and impedance matching is difficult.