How to perform source filtering with MultiSearch?

I have the following MultiSearch query:

{"_source": ["url"], "index": "profound"}
{"query": {"bool": {"should": [{"match_phrase": {"title": {"boost": 10, "query": "profound networks"}}}, {"match_phrase": {"": {"boost": 10, "query": "profound networks"}}}, {"match_phrase": {"text": {"boost": 5, "query": "profound networks"}}}], "must": {"match": {"text": {"query": "profound networks"}}}}}}

The query returns a list of entire documents. I'm trying to just get a part of the document back (the "url" field).

How do I achieve this? I can see how it works for MultiGet, but I can't find any filtering examples with MultiSearch.