A Hello World Apex HTTP Callout Example

DemoTriggerHandler.cls

public class DemoTriggerHandler {
     
    @future (callout=true)
    public static void sendTestRequest(string field1, string field2, string field3) {
        try{            
            HttpRequest request = new HttpRequest();
            request.setMethod('POST');            
            request.setEndpoint("my external api url goes here");            
            request.setBody('{"message":"Hello World"}');
            
            HttpResponse response = new HttpResponse();
            Http http = new Http();
            response = http.send(request);
            
            if (response.getStatusCode() == 200) {
                System.debug('I got a 200 OK from the external system');				
            }else{
                System.debug('Request Error: ' + String.valueOf(response.getStatusCode()) + response.getBody());
            }
            
            System.debug('completed');            
        }
        catch(System.CalloutException e){
            System.debug('Error-' + e.getMessage());   
        }
    }
}