Class

//Apex class to call external webservice, This class can be called using trigger
public class ActiveCampaignFieldUpdateCallouts {
    @future (callout=true)
    public static void UpdateActiveCompaign(String ContactIds){

        String username='';
        String password='';
        HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http();
        req.setHeader('Content-Type', 'application/x-www-form-urlencoded');
        req.setEndpoint('/admin/api.php?api_action=contact_edit');
        Blob headerValue = Blob.valueOf(username +':' +password);
        req.setMethod('POST');

        //req.setBody('userName=' +EncodingUtil.urlEncode(userName, 'UTF-8') +"&password=' + EncodingUtil.urlEncode(password, 'UTF-8') + '&cardNumber=' +cardNumber); // Request Parameters
        //req.setCompressed(true); // otherwise we hit a limit

        try{
            res = http.send(req);
             //check the response
            if(res.getStatusCode() == 200) {
                //update contact
                Contact conn = new Contact(id=ContactIds);
                conn.Description = res.getBody();
                update conn;
            }else{
                      System.debug('Callout failed: ' + res);
            }

        }catch(System.CalloutException e){
            System.debug('Callout error: '+ e);
            System.debug(res.toString());
        }

    }

}