vnktsf

String email = '<pardot email/username>';
String password = '<pardot password>';
String userKey = '<pardot user key>';

HttpRequest req = new HttpRequest();
req.setEndpoint( 'https://pi.pardot.com/api/login/version/4' );
req.setMethod( 'POST' );
req.setBody( 'email=' + email + '&password=' + password + '$&user_key=' + userKey );

HttpResponse res = new Http().send( req );