Use curl -i or even curl -v to see the response. I guess you’ll be seeing 401 Unauthorized:
You should not replace just the word secret, but all of ttn-account-v2.secret.
This works for me, though it defaults to port 0:
curl -i -X POST --data '{"dev_id": "arjanvanb-ttnode-1", "payload_raw": "dGVzdA=="}' https://integrations.thethingsnetwork.org/ttn-eu/api/v2/down/arjanvanb-app-ttnode/arjanvanb-http-ttnnode?key=ttn-account-v2.kFd<redacted>Vi_il4
HTTP/1.1 202 Accepted
Server: nginx/1.13.7
Date: Wed, 01 Jul 2020 10:20:50 GMT
Content-Type: text/plain; charset=utf-8
Content-Length: 0
Connection: keep-alive
Port 0 is reserved for MAC commands without an application payload, so you should also include "port": 1 or another value in the JSON data. Not doing so, it seems TTN will (currently) use the same port that was used for the uplink. But that’s undocumented behaviour, so surely it’s better to be explicit instead.
Note that the full downlink URL that you need is also in each uplink message that is posted by the HTTP Integration. So, you should see that on RequestBin.
(Aside, I know it’s just a test, but: don’t send text. Not even for a test.)