I want to create a text box in a form that contains a value that is hidden to the visitor but is added to the field when the visitor selects Submit.

I sell digital images online. I put the images in a subfolder on my Website and then email the buyer with the URL so they can retrieve them. Obviously they can click on the link in their email message and navigate to the folder, but I am looking for a more elegant way of doing that on my Website. Rather than having the buyer copy and paste or type the entire URL to their Web folder in the text field, I would like to be able to have them simply type in the name of their folder (henson1234) and when they press the Submit (or "Go" in this case) button the rest of the URL (http://www.xxxxxx.com/) will be added to the folder name and a new window will open displaying the contents of their folder.

Thanks in advance for any suggestions.