Hello,
Based on your message, I guess you want to test Browser or Ajax url ?
If you want to test URL, you can press F12 and check "Network" for XHR request.
If the window is not showing, you need to click outside of the game (but still on the preview game window)
And if you want to test ingame, just create your debug mod.
Put a text object on your game, and add some event that write logs you want.