trigger testvalidate on Account (before insert) {

        for (Account a : trigger.new)
        {
            ...

            HttpRequest req = new HttpRequest();
            req.setEndpoint('https://my url here ...');
            req.setMethod('POST');
            req.setBody(json);

            Http http = new Http();
            HttpResponse resp = http.send(req);

            ...
        }

}

estvalidate: execution of BeforeInsert caused by: System.CalloutException: Callout from triggers are currently not supported. Trigger.testvalidate: line 15, column 1