There's no set date for pre-order tickets to go on sale but Avengers: Endgame tickets went on sale in the month or so before the release date - so soon. Captain Marvel tickets went on sale January 7, that's two months before its March 8 release date. Endgame tickets could go live now Captain Marvel is out too. On the list that was shared to Reddit it said that Avengers: Endgame tickets go on sale April 2. Avengers: Endgame is out April 26, 2019.

Source: Daily Mirror March 12, 2019 18:22 UTC