To add BCC (blind carbon copy) in Outlook, follow these steps depending on your Outlook version:

For Outlook Desktop Application (Microsoft 365, 2021, 2019, 2016)

1. Open a new email by clicking New Email.
2. Go to the Options tab on the ribbon.
3. Click the Bcc button in the Show Fields section.
4. The BCC field will appear below the "To" and "Cc" fields.
5. Enter the email addresses you want to BCC in this field.
6. Compose and send your email as usual.

Once enabled, the BCC field will stay visible for new emails until you turn it off by clicking the Bcc button again

For Outlook on the Web or New Outlook

1. Click the gear icon (Settings) in the top right corner.
2. Select Mail in the left sidebar.
3. Click on Compose and reply.
4. Under the Message format section, check the box for Always show BCC.
5. Save changes.

Now the BCC field will appear by default every time you compose or reply to a message

For Outlook for Mac

1. Open a new email.
2. Click the Bcc button next to the To field to show the BCC line.
3. To always show BCC by default, go to Outlook menu > Settings > Composing.
4. Enable Show BCC field by default.
5. Add recipients to the BCC field and send your email

This method ensures recipients in the BCC field receive the email without their addresses being visible to others.