$customerSetup -> addAttribute(MagentoCustomerModelCustomer::ENTITY,
        'mobile',
        [
        'label' => 'Mobile Number',
        'system' => 0,
        'position' => 100,
        'sort_order' =>100,
        'visible' =>  true,
        'note' => 'this item used for login',


                    'type' => 'varchar',
                    'input' => 'text',

        ]
        );

    $customerSetup -> getEavConfig() -> getAttribute('customer', 'mobile')->setData('is_user_defined',1)->setData('is_required',1)->setData('default_value','')->setData('used_in_forms', ['adminhtml_customer', 'checkout_register', 'customer_account_create', 'customer_account_edit', 'adminhtml_checkout']) -> save();