Hi, i had a similar experience where clients needed to submit their details over the web and admin workers dealt with this using FileMaker Pro in the office. I was totally disappointed with the outcome of IWP. It is just a pretty story, i think, you cannot really use it for anything serious like the mentioned above. It is just not that easy (sorry to be the one that brings bad news). However, there are other better ways to integrate FileMaker Pro solution with your website. You might consider using CWP (Custom Web Publishing) methodology that would work much better for you. You might also do the web form using PHP-SQL platform and connect it with your FileMaker Pro database using ODBC driver for SQL data. I did this in my case and it works really great, people are loading the data using the website and the data is automatically processed inside the office database.
IWP, for the moment, in my opinion and experience, really does not look as an option for any kind of professional solution. I would praise anyone who would prove me wrong, it would be really cool to just have your forms done in FileMaker Pro and publish them as web pages to be used by the clients. Sounds to good to be truth. Maybe in FileMaker 12.
You can only specify a "page" in IWP, by making the link go directly to a database with the opening script to go to a layout and perform the script. This does not allow you to pass parameters...
fiddy, do you have static pages that otherwise could be accessed with an URL? can that be a field in your database then use OPEN URL[ ] script step to get to the page?