$referrer = $_SERVER['HTTP_REFERER'];

if( !empty( $referrer ) && !strstr( $referrer,'wp-login' ) && !strstr( $referrer,'wp-admin' ) ) {
if( $username == "" || $password == "" ){
    wp_redirect( get_permalink( 20 ) . "?login=empty" );
    exit;
}
}

wp_redirect( get_permalink( ID ) . "?login=empty" );

#1. wp_redirect( get_permalink( ID ) . "?members=" . $_GET["members"] . "?login=empty" );
#2. wp_redirect( get_permalink( ID ) . "?members=" . echo $_GET["members"] . "?login=empty" );
#3. $param = array('members');
wp_redirect( get_permalink( ID ) . "?members=" . $param . "?login=empty" );
//This one returns website.com/?members=array/?login=empty
#4.$param = $_SERVER["PHP_SELF"];
wp_redirect( $param . "?login=empty" );
// This one I cannot use because it displays the native login URL, not my permalink, that's why I need to use "get_permalink" for the first part of the URL

<input type="hidden" name="members" value="<?php echo @$_REQUEST['members']; ?>">

$queryarg='?login=empty';
if(isset($_REQUEST['members']) && $_REQUEST['members']!=''){
    $queryarg .='&members='$_REQUEST['members'];
}
wp_redirect( get_permalink( ID ) . $queryarg );

Parse error: syntax error, unexpected '$_REQUEST' (T_VARIABLE) on line 113.

$params_array = array();

foreach($_GET as $key => $param){
    $params_array[] = $key . '=' . urlencode($param);
}

$url = get_permalink(ID) . '?' . implode('&', $params_array);

wp_redirect( esc_url_raw( add_query_arg( 'login', 'empty', $url ) ) );

$params_array = array();

foreach($_GET as $key => $param){
    $params_array[] = $key . '=' . urlencode($param);
}

$url = get_permalink(ID) . '?' . implode('&', $params_array);

$args = array(
    'login' => 'empty'
    'param2' => 1
);

wp_redirect( esc_url_raw( add_query_arg( $args, $url ) ) );