User creation in web designer

Hi,
I am facing for the first time the Web Designer Tool. I need to configure the system so that only external users can be created from the Web Portal.
I made the field IsExternal read-only in the VI_Edit_Employee, but the field isExternal isn't flagged when I create a new user on the web portal.
Which parameter should I configure into the Node Editor to make isExternal true by default and not editable?

Thank you,

Elena