Hi @WillBurner-4298 ,
Thanks for reaching out.
As per the documention, using MS graph API you can only get these properties of a mailbox.
Since this feature/functionality is presently not available to get the GUID of mailbox using Microsoft Graph API, you can submit new feature request idea. This feature request portal is monitored by Microsoft team, and they make the enhancements to Microsoft Graph APIs based on the number of upvotes/API in demand. I will also upvote for you.
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote. If you have any further questions about this answer, please click Comment.