Every time I convert a user mailbox to a shared mailbox, it asks me to automatically answer with an already default preconfigured message.
Well, this message was set up when Outlook 365 was deployed and migration from Exchange was done and now, I'd like to change it because it is wrong written.
I already know I can change the message from each mailbox settings but what I'm looking for is the way to change pre-configured message in Outlook 365 admin portal so, each time I convert a user mailbox to a shared mailbox I'll get automatically pre-configured the right message.
Where exactly in the portal do you see this? Are you referring to the "delete user" wizard/flow? If so, try updating the message the next time you go over the steps, it should probably update. There is no configuration setting for this anywhere in O365, it's a purely UI thing.
Alternatively you can use the Feedback button on the lower right corner to report it.