It has been described in the second document. You need to add the field to the Add from too. It will look like the following MySQL commands:

INSERT INTO `sys_objects_form` (`object`, `title`, `action`, `form_attrs`, `submit_name`, `table`, `key`, `uri`, `uri_title`, `params`, `deleteable`, `active`, `override_class_name`, `override_class_file`) VALUES

('sample_form_objects', 'Sample Form Object', 'samples/form_objects.php', '', 'do_submit', 'sample_input_types', 'id', '', '', 'a:1:{s:14:"checker_helper";s:25:"BxSampleFormCheckerHelper";}', 1, 1, 'BxSampleForm', 'samples/BxSampleForm.php');

INSERT INTO `sys_form_displays` (`display_name`, `module`, `object`, `title`) VALUES

('sample_form_objects_add', 'sample', 'sample_form_objects', 'Add'),

('sample_form_objects_edit', 'sample', 'sample_form_objects', 'Edit');

INSERT INTO `sys_form_inputs` (`object`, `module`, `name`, `value`, `values`, `checked`, `type`, `caption`, `info`, `required`, `collapsed`, `html`, `attrs`, `attrs_tr`, `attrs_wrapper`, `checker_func`, `checker_params`, `checker_error`, `db_pass`, `db_params`, `editable`, `deletable`) VALUES

('sample_form_objects', 'custom', 'text', '', '', 0, 'text', 'Text', '', 1, 0, 0, '', '', '', 'avail', '', 'Text is required', 'Xss', '', 1, 1),

('sample_form_objects', 'custom', 'do_submit', '_Submit', '', 0, 'submit', '', '', 0, 0, 0, '', '', '', '', '', '', '', '', 1, 1);

INSERT INTO `sys_form_display_inputs` (`display_name`, `input_name`, `visible_for_levels`, `active`, `order`) VALUES

('sample_form_objects_add', 'text', 2147483647, 1, 20),

('sample_form_objects_add', 'do_submit', 2147483647, 1, 1001),

('sample_form_objects_edit', 'text', 2147483647, 0, 20),

('sample_form_objects_edit', 'do_submit', 2147483647, 1, 1000);