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Just to be sure I understand the question, I noticed that Brochure is one of the options in the dropdown on the form embedded in your webpage (http://www.kuche.co.uk/form.html). So, I take it you have a Request Brochure form which you would like to open in a lightbox when that option is selected. Is this correct? (I checked your JotForm account and there appears to be no Request Brochure form there. So, I'm assuming it's a form located somewhere else.)
For what it's worth, here are some guides relating to having our own forms open in a lightbox.
We'll await your reply on the exact type of functionality you're trying to implement on your webpage and then we'll see how best we can help you achieve it.