you cant do this in form builder, but you can add it after the form is created.
in form builder add the email and confirm email fields,
after the form is created, edit the Server validations server behavior to add a new server validation, use the Like Entry validation type, select the email field as the server variables, when you add it to the validation list, you will be prompted for the comparison select the confirm email element.
you will also need to add the validation error message, and validation show if behavior to show the error when validation fails.