While the rebase is going, the autostash is stored in : .git/rebase-merge/autostash.
Since this file is stored under .git/, and is a valid reference file, you can use rebase-merge/autostash as a name to point to the autostash :
git log --oneline --graph rebase-merge/autostash
# you can also use it with 'git stash show' or 'git stash apply' :
git stash show rebase-merge/autostash
git stash apply rebase-merge/autostash
You can also "insert" it in the regular stash by manually calling git update-ref refs/stash :
git update-ref -m "autostash" refs/stash rebase-merge/autostash
note : before reading your question I also thought the autostash was as a regular stash, I just found out about the tricks above by inspecting the content of .git/rebase-merge/.
The hacks above work at the moment (git version 2.31), I don't think you should count them as a rock solid, forever stable feature -- for example the internal structure of the .git/rebase-merge/ directory isn't part of the documentation.
and obviously, if you run git rebase from a terminal, you can also see the hash of the autostash in the command's output :
$ git rebase -i master
Created autostash: 2acfb51
...