In Airtable you can generate a sharing link that requires a password to access (more secure than the normal link where anyone with the link can see the whole base) - we can now support them.
When you are setting up the portal, at the "Connect your Airtable" step, there is a new link "My shared base link is password protected". Click on this link which will open a "Your shared base password"
Fill in the password there and everything should work as normal.
If you ALREADY have a portal and would like to ADD a password:
Setup home → Portal settings → "Password protected Airtable base"
Fill in and save your password from there.