<?php
    namespace vendormoduleObserver;
    use MagentoFrameworkEventObserverInterface;

    class customeramount implements MagentoFrameworkEventObserverInterface
    {
     public function __construct(MagentoFrameworkAppConfigScopeConfigInterface $scopeConfig )
        {
         $this->_scopeConfig = $scopeConfig;
        }
    public function execute(MagentoFrameworkEventObserver $observer)
      {
        $objectManager = MagentoFrameworkAppObjectManager::getInstance();
         $customerSession = $objectManager->create('MagentoCustomerModelSession');

     if($customerSession->isLoggedIn())
     {
       $customer_id = $customerSession->getCustomer()->getId();
       $customerSession->setMemebrCustomAttr('abc');

    }
  }

}
?>