As far as I know, Office 365 Shared Mailboxes usually don't require a license and have no SMTP access to Exchange online. If you would like to send mail using SMTP AUTH client submission, then you must have a licensed Microsoft 365 or Office 365 mailbox to send email from.
For more information about options available to setup third-party hosted application or devices to send email using Microsoft 365 or Office 365, hopefully you can find the document below helpful:
How to set up a multifunction device or application to send email using Microsoft 365 or Office 365
You may also have a look at the following link which discusses a relevant question:
Can Office365 shared mailbox use SMTP?
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.