Use below code for your layout customer_account_login.xml file
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="customer_form_login">
<action method="setTemplate">
<argument name="template" xsi:type="string">Magento_Customer::form/login123.phtml</argument>
</action>
</referenceBlock>
</body>
</page>