The 19-year-old striker was one of four new faces named in Billy Stark's 18-man squad for next Wednesday's match against Greece in Corinth, with the Scottish Football Association using its own Twitter account to break the news.
And ahead of tomorrow's Scottish Cup fifth-round tie against Inverness at Rugby Park, McKenzie, who represented Scotland at under-19 level, told how he was one of the last to know about his step up.
"I'm not on Twitter so all the boys knew before me," he said. "I'd just come in from training and the boys told me.
"I'm about the only one who isn't on Twitter. I can't be bothered with it, I'm on Facebook but Twitter, it's too much for me.
"It was a bit of a surprise, because I wasn't involved in the last few camps, which I was a bit disappointed about. I've been playing the last couple of weeks in the first team, so it's very pleasing."
McKenzie scored in his first start for Killie in the 3-1 win against Dundee United at Tannadice in August and is glad that boss Kenny Shiels is giving youth a chance at the Ayrshire club.
"It was a good start to the season for me with getting the goal against Dundee United," he said. "I was playing a lot but it has been quite a bit stop-start since then.
"I started against Dundee last week but was disappointed to be left out against Celtic in midweek, but hopefully I start again (against Inverness).
"The manager is not scared to throw young players in which is great. We've got a good youth team and it's good knowing that you will get a chance.
"We were knocked out early on (in the Scottish Communities League Cup) at Stenhousemuir so we've got a point to prove."
Shiels believes McKenzie's call-up is a "great honour" for the club.
He said: "It's fantastic. It is another part of how we are progressing as a club; we are capped at every level, at 15's, 17's and 19's, it's great.
"There's lots of prospects coming through."
However, Shiels, who took Killie to League Cup glory last year, is aware that Caley will be a tough nut to crack.
"There's 16 teams left in the competition and there are 16 teams who think they can win it," he said. "It's a tournament steeped and rich in history of small clubs going to Hampden, and I'd like us to be one of them.
"Queen of the South went to the final, Gretna went to the final. If they had gone out with a mindset that they can't win it, then they wouldn't have got to the final.
"But it's a long way away, we have got to try and get through Inverness and everyone knows how well they are going."