You can change default signup fields (required or non required). The best example for this hook usage is Singup fields component that adds the mobile number field to the signup

function com_signup_fields_mobile_init(){
ossn_add_hook('user', 'signup:fields', 'com_signup_fields');}
function com_signup_fields_mobile($hook, $type, $fields){
$label = ossn_print('com:signupfields:mobile:label');
$placeholder = '';

$extrafield = array(
'name' => 'com:signupfields:mobile',
'label' => $label,
'placeholder' => $placeholder,
'display_on_about_page' => true
//for nonrequired change it to `non_required`
//$fields contains fields from previous components
//so don't change return type $fields.
//if you notice $field is also the 3rd argument of function
$fields['required']['text'][] = $extrafield;
return $fields;
ossn_register_callback('ossn', 'init', 'com_signup_fields_mobile_init');

User signup fields