You can use following code
add_filter( 'fep_form_fields', function( $fields ){
$fields['cus_key'] = array(
'type' => 'checkbox',
'value' => fep_get_user_option( 'cus_key', 1),
'cb_label' => __("Your Label", 'front-end-pm' ),
'priority' => 35,
'where' => 'settings',
);
return $fields;
});