Figuring out what the Developer Tool service Call actually do

Hello I am using the developer tool services to play YouTube on my chrome cast, the call from he HA developer tool works fine, but when I do an external Restful post to the HA service call, it comes back as 400: bad request. BTW I have been using external REST POST to other services and it works well but can’t get the external service call to chrome cast to work correctly. So in trying to debug it, I thought I use Wireshark, But I can only see the external REST request, but see no http traffic from the Developer Tools service call.

So my question is how do I start look into what the developer tool service calls do? For example when I do a post request to http://homeassistant.local:8123/api/services/media_player/play_media?

alternatively, I am trying to figure out how to pass the media_content_id correctly to a HA REST call in a an external REST POST to HA