To stash your working directory including untracked files (especially those that are in the .gitignore) then you probably want to use this cmd:
git stash --include-untracked
Alternatively, you can use the shorthand -u instead of --include-untracked, or simply git stash --all which stashes all files, including untracked and ignored files. This bahaviour changed in 2018, so make sure your git is up to date.
Warning: there seems to be (or have been) situations in which contents of ignored directories could be deleted permanently. See this archived website for more information.