How to bypass the IWP login page

Since v8 FileMaker® Server Advanced got a new IWP login page.

It's better than the previous authentication scheme in a number of ways. The most important one is that you can use non-romanized account names for non-English applications. If you application is entirely in Russian or Japanese, romanized account names look quite awkward. With FileMaker 8 romanized ‘Petrov’ and ‘Ueda’ can be changed to real ‘Петров’ and ‘上田’.

Yet it also has some disadvantages. Though you can customize the IWP home page, there's no instructions on how you can place the authentication form on this page and save your users a few clicks. If you want to allow both guests and authenticated users, guests will have to go through the same authentication page as well. The form is implemented in such a way that users can no longer have their browsers to remember the login and password. Unless they use some other password management utility like KeePass, they will have to type both their login and password every time. As I see this is intentional (the form has the autocomplete attribute explicitly set to off), but such a strict policy is not always necessary.

On FileMaker Forums I've seen some requests about this problem. There are also answers, but none of them gives a complete solution and some suggestions are a bit too complex. So here are exact recipes that work.