To send emails FROM a shared mailbox, you'll need to change the email in the from box. In the Outlook App, if this option doesn't appear by default it can be enabled under Options as seen below:
Click 'Other Email Address' then enter the shared mailbox address to add it to the drop down list.
To add a signature that will be used to send from this account, you will need to first add it as an account on your outlook.
Select File - > Add account
Enter the shared mailbox address
As there is no password for the shared mailbox you will need to select sign in with another account
Then enter your primary email address
Proceed with signing in with your primary email credentials and validate with the MFA on your account, then restart Outlook.
Then to add the signature navigate to Insert -> Signatures -> Signatures. Choose the shared mailbox then select new and enter the signature in the body of the 'Edit Signature' section. You can set defaults for new messages and replies here.
