I think it will be somewhere in here. It will probably get installed on your PC with this thing.
https://github.com/microsoft/GDK
edit: you can see references to xurl.dll and xurl.h here, which is GDK examples
https://github.com/search?q=repo%3Amicrosoft%2FXbox-GDK-Samp...
so the xcurl.dll and xurl.h is really probably in the installer.
I think it will be somewhere in here. It will probably get installed on your PC with this thing.
https://github.com/microsoft/GDK
edit: you can see references to xurl.dll and xurl.h here, which is GDK examples
https://github.com/search?q=repo%3Amicrosoft%2FXbox-GDK-Samp...
so the xcurl.dll and xurl.h is really probably in the installer.