Megapop’s answer is pretty much right. However I’d advice to use position FIXED rather than ABSOLUTE ( unless you want your popup to be able to scroll off the viewport).
For a small confirm- or info-popup I’d agree, but since the OP has an entire form (which is supposed to be responsive and thus labels and inputs might get stacked beneath each other on mobile devices) this deemed me the most general solution w/o further media-queries, max-height, overflow-y: scroll etc…
Edit: Having a closer look, this form should easily fit within the viewport anyway… so yes, you’re probably right! :~)
Hey applytobecome, I used the script on a sample page and it worked fine. The fallback-link-to-form-page is just a way to reset the form page. It basically just refreshes the page to show just the contact link without the form. What is the problem you are having with making the page work?
If it’s a standard website you are building from scratch, you can check 123contactform, you create the form and you get the lightbox code.
If you plan on using it on Wordpress you should try our plugin which is CaptainForm
Certain elements of a website are just not worth going through the trouble of coding them from scratch when there’s builders out there that will do it for you, forms are one of them