Create di.xml and add the below code
Magemonkey/Redirect/etc/frontend/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
  <type name="MagentoCustomerControllerAccountLoginPost">
    <plugin name="redirect_custom_url" type="MagemonkeyRedirectPluginRedirectCustomUrl" sortOrder="1" />
  </type>
</config>

Create the plugin
Magemonkey/Redirect/Plugin/RedirectCustomUrl.php

<?php
namespace MagemonkeyRedirectPlugin;
class RedirectCustomUrl
{
    public function __construct(
        MagentoStoreModelStoreManagerInterface $storeManager,
        MagentoFrameworkAppHttpContext $authContext,
        MagentoFrameworkAppResponseRedirectInterface $redirect,
        MagentoFrameworkAppRequestInterface $request        
    ){

        $this->storeManager = $storeManager;
        $this->authContext = $authContext;
        $this->redirect = $redirect;
        $this->_request = $request;        
    }
    public function afterExecute(MagentoCustomerControllerAccountLoginPost $subject,$result)
    {
        $currentWebsiteId = $this->storeManager->getStore()->getWebsiteId();
        $isLoggedIn = $this->authContext->getValue(MagentoCustomerModelContext::CONTEXT_AUTH);
        if ($isLoggedIn){
            $customUrl = 'customurl.html';
            $result->setPath($customUrl);
    	}
        return $result;
    }
}