Turns out some people over at FreeRDP were faster figuring out how the connection is established than Microsoft providing some information. see https://github.com/FreeRDP/FreeRDP/issues/6529#issuecomment-1605456423 and following posts
Of course there is no support for "shortpath", but that does probably also not work in the web browser version.
It would still help if microsoft would document the new feed subscription model and the extensions they made in the rdp protocol (ex this https://github.com/FreeRDP/FreeRDP/pull/9093/commits/e5684e40eaf136e4b5fc7278b4f2b2956cf78f61 )
Don't really get why they don't provide information since they earn by hosting the avd, not the client. But without the client the whole thing is useless. A webbrowser is not really a replacement for a native client