<?php
use Microsoft\Graph\Beta\GraphServiceClient;
use Microsoft\Graph\Beta\Generated\Models\Security\UnifiedGroupSource;
use Microsoft\Graph\Beta\Generated\Models\Security\SourceType;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestBody = new UnifiedGroupSource();
$requestBody->setIncludedSources(new SourceType('mailbox'));
$additionalData = [
'group@odata.bind' => 'https://graph.microsoft.com/v1.0/groups/93f90172-fe05-43ea-83cf-ff785a40d610',
];
$requestBody->setAdditionalData($additionalData);
$result = $graphServiceClient->security()->cases()->ediscoveryCases()->byEdiscoveryCaseId('ediscoveryCase-id')->custodians()->byEdiscoveryCustodianId('ediscoveryCustodian-id')->unifiedGroupSources()->post($requestBody)->wait();