Pre Defined data in form fields

Hi, I am interested in placing an enquiry form on my website which will add data to the database. The user can fill in name, contact details etc. But I need a way to populate a field with a reference Number to a particular Vaction rental property so I know what property they enquire about. I cannot rely on the user putting in their own subject line.

The html code for embedding the form into a website does not allow for this so it seems.

I have tried using formmail and adding data via email but this does not seem to work via formmail.

Any ideas anyone?