Custome Controller:
public class RecallOperationalApprovalController {
    @AuraEnabled
    public static void updateQuote(Id recordId)
    {
        SBQQ__Quote__c updatedQuote = [SELECT Id, Recall_Ops_Approval__c FROM SBQQ__Quote__c WHERE id = :recordId];
        updatedQuote.Recall_Ops_Approval__c = true;
        update updatedQuote;
    }
}

UseCase. Whenever an opportunity is Closed Won In Salesforce, a notification should be sent to a slack channel.
Steps i already completed:
Created apex class and test class 
Created a process builder to run that apex class with the reuired consition.
Error i am getting:
Developer script exception from Numerator : SlackOpportunityPublisher for job ID 7071V00003XTnvR. : Unauthorized endpoint, please check Setup->Security->Remote site settings. endpoint = https://xyz.slack.com/services/T0298VAEK/BLPETK2R2/Rac52NjPIQSkanrlNAV8r5BL

Not able to add this full endpoint. Can someone please help.
Thanks.

Custome Controller:
public class RecallOperationalApprovalController {
    @AuraEnabled
    public static void updateQuote(Id recordId)
    {
        SBQQ__Quote__c updatedQuote = [SELECT Id, Recall_Ops_Approval__c FROM SBQQ__Quote__c WHERE id = :recordId];
        updatedQuote.Recall_Ops_Approval__c = true;
        update updatedQuote;
    }
}