Apex Class which retrieves the Survey_URL__c field from the LiveChatTranscript with the specified ChatKey

SurveyRedirectController.apxc

public class SurveyRedirectController {
	public PageReference redirectToSurvey() {
		String url = [SELECT Id, Survey_URL__c, ChatKey FROM LiveChatTranscript WHERE ChatKey = : ApexPages.currentPage().getParameters().get('chatKey')].Survey_URL__c;
		PageReference surveyURL = new PageReference(url);
        return surveyURL;
    }
}