var requestBody = new QueryPostRequestBody
{
	Requests = new List<SearchRequest>
	{
		new SearchRequest
		{
			EntityTypes = new List<EntityType?>
			{
				EntityType.ExternalItem,
			},
			ContentSources = new List<string>
			{
				"/external/connections/connectionfriendlyname",
			},
			Region = "US",
			Query = new SearchQuery
			{
				QueryString = "contoso product",
			},
			From = 0,
			Size = 25,
			Fields = new List<string>
			{
				"title",
				"description",
			},
		},
	},
};

var result = await graphClient.Search.Query.PostAsQueryPostResponseAsync(requestBody);

const searchResponse = {
  requests: [
    {
      entityTypes: [
        'externalItem'
      ],
      contentSources: [
        '/external/connections/connectionfriendlyname'
      ],
       region: 'US',
       query: {
        queryString: 'contoso product'
      },
      from: 0,
      size: 25,
      fields: [
        'title',
        'description'
      ]
    }
  ]
};

await client.api('/search/query')
	.post(searchResponse);