$customerSetup->addAttribute(
  Customer::ENTITY, 'company_name', [
        'type' => 'varchar',
        'label' => 'Company Name',
        'input' => 'text',
        'required' => true,
        'visible' => true,
        'user_defined' => true,
         'backend'  => '',
         'frontend'=>''
    ]);
    $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'company_name')
    ->addData([
        'attribute_set_id' => $attributeSetId,
        'attribute_group_id' => $attributeGroupId,
        'used_in_forms' => ['adminhtml_customer','customer_account_create','customer_account_edit'],
    ]);