ClickHouse/tests/integration/test_storage_mongodb/configs/named_collections.xml

<clickhouse>
    <named_collections>
        <mongo1>
            <user>root</user>
            <password>clickhouse</password>
            <host>mongo1</host>
            <port>27017</port>
            <database>test</database>
            <table>simple_table</table>
        </mongo1>
    </named_collections>
</clickhouse>