$customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);

        $customerSetup->addAttribute(MagentoCustomerModelCustomer::ENTITY, 'custom_attr', [
            'type' => 'varchar',
            'label' => 'Customer Position',
            'input' => 'text',
            'source' => '',
            'required' => false,
            'visible' => true,
            'position' => 999,
            'system' => false,
            'backend' => ''
        ]);
        $attribute = $customerSetup->getEavConfig()->getAttribute('customer', 'custom_attr')
            ->addData(['used_in_forms' => [
                'adminhtml_customer'
            ]]);
        $attribute->save();