About:

This is how to add shared mailbox to your phone mail app. You must have full access to the shared account to be able to do that

The steps here are shown on iPhone mail app.

Steps:

You have to add it as IMAP account, and use the Advanced Settings:

Name : whatever you want

E-mail: sharedmailbox@domain.com

IMAP:

Host: outlook.office365.com

Username: your_primary_login@domain.com\sharedmailbox

Password: your pass

SMTP

Host: smtp.office365.com:587

Username: your_primary_login@domain.com

Password: your pass