Sorry I forgot to get back to you, but good work figuring that out.
By the way, I have developed a bunch of code for Home Assistant by simply performing the action in Chrome with the web tools capturing each web call… for example manually clearing the existing queue in Synology Audio Centre, and copying the web call as a curl command. I can explain further if you want more detail.
Also you can find the Synology API manuals, they are pretty good once you get the hang of their approaches.