Before any official announcement, Paul Rudd dropped the "Endgame" title on the low.
Paul Rudd is mischievous to say the least. Known for his trollish behavior on many-a late night stage, Rudd has a recurring history of implementing clips of Mac And Mee into his otherwise informative interviews. Though he did manage to abstain from such shenanigans during his last visit to Seth Mayer’s, that didn’t stop Ant-Man from some low key trolling. Trolling that, in all likelihood, left Marvel higher-ups in shambles.
Long before Avengers Endgame was revealed as the culminating film’s title, many eagle-eyed fans kept a lookout for hints and Easter eggs. As it happens, both Rudd and Seth dropped the reveal on the low, as you can see through the 20-20 benefits of hindsight. Rudd opened up about the vow of silence that comes with a property such as The Avengers, as well as the difficulties of staying mum's the word. The conversation prompted Myers to say "I don’t know how that could mess up, though, because I feel like that would make more people see the movie, which is the endgame.”
Seth Meyers leaked the Avengers Endgame title back when Paul Rudd was promoting Ant-Man and the Wasp from r/marvelstudios
Were Rudd to have had a mustache, he might have twirled it.“You know what?" he replied. "You’re right." All that’s missing is a wink-wink-nudge-nudge, though it’s implied nevertheless. Now that the title is out in the open, it’s safe to say that Ant-Man will play a pivotal role in righting some of Thanos’ wrongdoings, be it through knowledge of the quantum realm, witty banter, or both.