In order to create quick step in a shared mailbox, you need to access said mailbox in direct mode, either via the Open another mailbox functionality in OWA or via File > Add account in Outlook. Both methods require that you have full access permissions on the mailbox. If you need detailed instructions, check for example this article: https://www.michev.info/blog/post/3567/how-to-add-a-shared-mailbox-as-additional-account-in-outlook-2022-version
Once you do this, you will be able to work with Quick steps, categories, rules and any of the other features Outlook supports, just like with a "regular" account.