Event observer – Update custom data for the client

I'm trying to create a new client attribute "last_login_at" to store the time of the client's last session. My requirement is to update "last_login_at" data for the client after the client has logged in. I really do not know how to update it with an observer. Can anybody help me?
Thank you,

Here is my code to create the attribute:

if (version_compare ($ context-> getVersion (), & # 39; 2.2.17 & # 39 ;, & # 39;<')){
        $eavSetup = $this->eavSetupFactory-> create (['setup' => $setup]);
$ attributeCode = & # 39; last_login_at & # 39 ;;


$ eavSetup-> addAttribute (
 Magento  Customer  Model  Customer :: ENTITY, // customer entity code
& # 39; last_login_at & # 39 ;,
            [

                'label' =>"Last Login",
                'type' => 'datetime',
                'input' => 'date',
                'frontend' => MagentoEavModelEntityAttributeFrontendDatetime::class,
                'backend' => MagentoEavModelEntityAttributeBackendDatetime::class,
                'required' => false,
                'user_defined' => true,
                'visible' => true,
                'system' => false,
                'input_filter' => 'date',
                'validate_rules' => '{"input_validation":"date"}',
                'position'     => 999,


            ]
        );

$ eavSetup-> addAttributeToSet (
 Magento  Customer  Api  CustomerMetadataInterface :: ENTITY_TYPE_CUSTOMER,
 Magento  Customer  Api  CustomerMetadataInterface :: ATTRIBUTE_SET_ID_CUSTOMER,
null,
& # 39; last_login_at & # 39;);

$ lastLogin = $ this-> eavConfig-> getAttribute ( Magento  Customer  Model  Customer :: ENTITY, & # 39; last_login_at & # 39;);


$ lastLogin-> setData (
& # 39; used_in_forms & # 39 ;,
            ['customer_account_edit','adminhtml_customer','customer_account_create'] 
        );
$ lastLogin-> save ();
}
$ setup-> endSetup ();
}