Add a Office 365 Shared mailbox on an iPhone using IMAP

Go to Settings >

Add Account > Other

Add Mail Account
    Name: [Users Name]
    Email: [Shared Mailbox Email Address]
    Password: [Users Main 365 Password]
Next

Incomeing Settings –
    Hostname: outlook.office365.com
    username: [Users main email address]/shared mailbox name before the @ sign]
    Password: [Users Main 365 Password]
    
    Outgoing Settings –
    Hostname: smtp.office365.com
    Username: [Users main 365 email address]
    Password: [Users Main 365 Password]
Next
Email Account has now been setup