The Hammers went down 2-1 at London Stadium on Thursday night.
"Today probably wasn't one of our better performance but one thing we like about this is it's two legs," Antonio told the club's website. "This was just the first leg and next week we've got to go again, so it's all to play for. We've been in this situation before with Sevilla so we've got to go there, put in the work, and you never know.
"It was a slow start. One goal down. It's not how you want to begin the game but, from there we carried on, we got playing well, we got control of the game and created a few chances. We didn't take them today but it's not over."
He added: "The overriding feeling is we're here to win it. We're in it to win it. We've got another game to go.
"One thing for us is no matter where we go, we believe we can beat anybody. We're going to go there and try to win."
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