To have access to this field, make sure you allow both user and site registration in Network Settings.
The Default – Blog Details field only appears on the Registration Page allowing the creation of a sub-site. Inside the field options you can set the field title and description.
Users can choose to create a new subsite along with their registration. In this case, they need to fill in the Site slug, Site title and Privacy option.
This is how the Default – Blog Details field we created above looks like in the Registration page in front-end.