public class LongRunningCallout {
    // Declare requestLabel and result as class variables
    public String requestLabel {get; set;}
    public String result {get; set;}
    
    public Object startRequest() {
        // Create continuation with a timeout
        Continuation con = new Continuation(60);
        con.continuationMethod = 'processResponse';
        
        // Create callout request
        HttpRequest req = new HttpRequest();
        req.setMethod('GET');
        req.setEndpoint('https://jsonplaceholder.typicode.com/posts');
        
        // Add callout request to continuation
        this.requestLabel = con.addHttpRequest(req);
        
        // Return the continuation
        return con;
    }
    
    // The callback method that processes the callout response
    public Object processResponse() {
        // Get the response by using the request label
        HttpResponse response = Continuation.getResponse(this.requestLabel);
        result = response.getBody();
        System.debug('Response-result:' + result);
        // Process the response
        System.debug('Response: ' + response.getBody());
        
        // You can return null or a PageReference here
        return null;
    }
}