The Hammers' usual penalty taker and captain Mark Noble was on the pitch when Paul Pogba blocked Declan Rice's free kick with his hands to gift the away side a spot kick, which Antonio duly converted.
The game finished 1-1 after Mason Greenwood equalised for the Red Devils, but the result means West Ham are now safe from relegation.
Moyes, speaking in the post-match press conference as reported by Football.London, insisted the decision was based on Antonio's recent form.
“Mark Noble is the normal penalty taker but actually felt today [Wednesday] that Micky Antonio was the right choice," said Moyes.
"His goals have been big for us, we probably wouldn't have been in such a good position without his goals so I had no problem with it at all. We practiced them the other day in training as well so I had a feeling that he may well take one if we got one."
Michail Antonio certainly saw the funny side of Paul Pogba's handball! ?— Sky Sports Premier League (@SkySportsPL) July 22, 2020
The Frenchman did not look impressed... ?
Report as #WHUFC secure their Premier League safety with Old Trafford draw: https://t.co/WfrHcIMHZi pic.twitter.com/eKdInua01H
No one has scored more goals than Antonio since the restart. The 30-year-old has now hit eight of his ten league goals this season since the Premier League returned in May.
Speaking to the club's website Antonio added: "Me and Nobes talked and he asked me what my target was for the season. I said it’s ten goals and it’s been my target for the last five years I’ve been here.
"I’ve never been able to reach it - I’d got to nine a couple of times but not ten - so he said if we get a penalty in the next two games he’d give it to me because I’d scored so many important goals for us this month and he believed I would score."
"I honestly didn’t think we’d get a penalty, and when we got it, I didn’t think he’d still give it to me! But as soon as they made the decision on VAR he said: ‘Are you ready?’ And I said; ‘Yes I am.’ I knew where I was going to put it. Because he spoke to me in the week I had a couple of practice penalties, so because I knew where I was going to put it I was confident, and just slotted it home."
Despite finishing the spot-kick well, the forward said it will be back to normal on Sunday when West Ham host relegation threatened Aston Villa.
He added: "Nobes will be back on them from Sunday, this was literally so I could hit my target of ten goals for the season and hopefully I can score another in open play!"
Source : 90min