Triggering all build agents from the API is no longer working since TC 2017

On the previous setup (TC 9) I triggered a maintenance task by having one agent sending the http get request from the API, unfortunately this did no longer work after upgrading to TC2017.2 as the request was changed to a http post and it's now sending xml data as content. 

Not a real problem, I just wrote a small app that does exactly that; take the info and send a post rather than a request. It does work if I keep exactly to the manual and send the trigger in general or to a specific agent. I tried to use 

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<build>
<buildType id="UpdateAgent"/>
<agent id="allEnabledCompatible"/>
</build>

To trigger it for all agents but that doesn't work and i get a Bad request (400) as response.

How do I make this trigger all (enabled compatible) agents?