As was pointed out in the comments: Your JSON formatting is off, so the API doesn't understand that you're asking for the URL.
For an understanding of what's going on, take a look at the example request in the link you provided; note the format of the fields section.
Try again, this time with:
{"survey_id": "47625442", "fields": ["url"]}
...which with the escapes will look like this....
{\"survey_id\": \"47625442\", \"fields\":[ \"url\" ]}"