Put an OnObjectEnter script trigger on the field. Set it to perform a script that uses Show Custom Dialog with a global field specified as an input field in the custom dialog. There's an input field option that will replace the characters entered with bullets.
Your script might look like this:
Show Custom Dialog ["Please enter manager approval password:"]
IF [YourTable::gPasswordField ≠ "Open Sesame"]
Show custom Dialog ["invalid password"]
Go to field [//specify another field here or use go to object to put the focus on a layout object with an object name]
It's also possible for the script to use either get ( accountName ) or get ( accountPrivilegeSetname ) to identify whether the current user should be permitted access to the field. In this case, you don't have to use show custom dialog to ask for a password.