<?php
use Microsoft\Graph\Beta\GraphServiceClient;
use Microsoft\Graph\Beta\Generated\Models\Security\SiteSource;
use Microsoft\Graph\Beta\Generated\Models\Site;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestBody = new SiteSource();
$requestBody->setOdataType('microsoft.graph.security.siteSource');
$site = new Site();
$site->setWebUrl('https://m365x809305.sharepoint.com/sites/Design-topsecret');
$requestBody->setSite($site);
$result = $graphServiceClient->security()->cases()->ediscoveryCases()->byEdiscoveryCaseId('ediscoveryCase-id')->searches()->byEdiscoverySearchId('ediscoverySearch-id')->additionalSources()->post($requestBody)->wait();