The Robins clinched a top-six spot with a 3-0 home win over Stevenage, who were forced to play with 10 men for 50 minutes following Dani Lopez's dismissal.
Defender Aden Flint scored twice, along with Gary Roberts.
McDonald stressed his side will not be sitting back as they head to Scunthorpe for their last game of the season with their play-off berth sealed.
He said: "We need to keep the momentum going by getting the right result at Scunthorpe, but then every other manager will be saying that."
McDonald admitted his side's cause was assisted by Lopez's red card five minutes before half-time.
Referee Gary Sutton was left with little option but to apply the letter of the law as Lopez raised his hands and appeared to slap Swindon skipper Alan McCormack.
McDonald said: "We were helped by the sending off. We were a little sloppy at times in midfield but we've now had a couple of good solid performances behind us."