eDiscovery.ReadWrite.All

const options = {
	authProvider,
};

const client = Client.init(options);

const dataSource = {
    '@odata.type': 'microsoft.graph.security.siteSource',
    site: {
        webUrl: 'https://contoso.sharepoint.com/sites/SecretSite'
    }
};

await client.api('/security/cases/ediscoveryCases/{ediscoveryCaseId}/searches/{ediscoverySearchId}/additionalSources')
	.post(dataSource);

Import-Module Microsoft.Graph.Security

$params = @{
	"@odata.type" = "microsoft.graph.security.siteSource"
	site = @{
		webUrl = "https://contoso.sharepoint.com/sites/SecretSite"
	}
}

New-MgSecurityCaseEdiscoveryCaseSearchAdditionalSource -EdiscoveryCaseId $ediscoveryCaseId -EdiscoverySearchId $ediscoverySearchId -BodyParameter $params

# Code snippets are only available for the latest version. Current version is 1.x
from msgraph import GraphServiceClient
from msgraph.generated.models.security.site_source import SiteSource
from msgraph.generated.models.site import Site
# To initialize your graph_client, see https://learn.microsoft.com/en-us/graph/sdks/create-client?from=snippets&tabs=python
request_body = SiteSource(
	odata_type = "microsoft.graph.security.siteSource",
	site = Site(
		web_url = "https://contoso.sharepoint.com/sites/SecretSite",
	),
)

result = await graph_client.security.cases.ediscovery_cases.by_ediscovery_case_id('ediscoveryCase-id').searches.by_ediscovery_search_id('ediscoverySearch-id').additional_sources.post(request_body)