// dispatch message. note 'GameObject' is euphemism for whatever game object will have the script attached to it.
		if (Button ("Get Invoking URL"))
		{
			Application.ExternalEval(
				"u.getUnity().SendMessage('GameObject', 'OnInvokingParamsResponse', window.location.href);"
			);
		}

        // assuming this script is attached to 'GameObject' this function will be invoked
	public void OnInvokingParamsResponse(string res) {
		Debug.Log("invoking params res: "+res);
	}

quick example of how to get invoking URL in unity

getting invoking url and params

C#