What you want to use as an app key is completely up to you (as long as it doesn’t contain invalid characters). As in the example, you could do something like:
com.yourcompany.confluence.codex-for-team
But you could also choose a less complex key and go with something simple like:
codex-for-team
Be aware however that if you want to create a version of your app for Jira or Bitbucket as well later on, that this will require a different listing. So in that case you’ll need a different key. For that reason, it is not a bad idea to include the host product (i.e. jira or confluence) in your app key somehow.