Give visitor access to password protected page/post via external script

Finally, I found the solution:

form.php

<form action="<?php echo bloginfo('template_directory'); ?>/pp-redirect.php" method="post">
Event: <input name="event" type="text" size="25" /><br />
Passwort: <input name="post_password" type="password" size="25" /><br />
<input name="submit" type="submit" value="Los" />
</form>

pp-redirect.php

<?php
/** Make sure that the WordPress bootstrap has run before continuing. */
require( dirname(__FILE__) . './../../../wp-load.php');
global $wp_hasher;
if ( empty( $wp_hasher ) ) {
    require_once( ABSPATH . 'wp-includes/class-phpass.php' );
    $wp_hasher = new PasswordHash(8, true);
}
if ( get_magic_quotes_gpc() )
    $_POST['post_password'] = stripslashes($_POST['post_password']);

// Expires when the browser shuts down
setcookie( 'wp-postpass_' . COOKIEHASH, $wp_hasher->HashPassword( stripslashes( $_POST['post_password'] ) ), 0, COOKIEPATH );

wp_safe_redirect( get_bloginfo('url') . "https://wordpress.stackexchange.com/" . $_POST['event'] );
?>

Greets!