Post-functions are not "after a transition", they are the last set of things executed as a transition runs, they are very much part of a transition.
No, it's too early to have one in the post-functions of a create process. You can't know it until the issue has been created, and it won't be until after the "create issue initially". When you are running post-functions, they run in the context of the issue that was set before the issue was created, when you don't know the issue key. If you look at the default post-functions, you'll see indexing and event firing post-functions which do not work off the issue key (because they don't have one). You could try to replicate how they identify the issue.
I would suggest using a listener instead - the "issue created" event will point to the newly created issue which will, by then, have a key.
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