Which suggests that you need to use a canning (mason) jar and canning lid, not over-tightened - i.e., equipment made to handle the pressure and heat, not just a mayo jar you happen to have lying around.
This will effectively be a "water bath canning" procedure -- but do NOT think that the milk is properly preserved (canned), even though the lid will probably seal. Milk that has not been allowed to acidify is not low enough pH to prevent botulism, and hot-water-bath canning is not high enough heat to kill off the spores.