I think the OP wants the website to describe the business that will be "coming soon" rather than have an "under construction" page.
I don't see why you can't simply edit the page to make it clear that you've not yet commenced trading, but these are the products/services you will provide and here is how a potential customer to contact you, subscribe to a mailing list or register their interest so you can let them know when trading commences, etc. etc.
Just be clear about this in the text of your home page. It's an acceptable practice, and far better than having just a "under construction" (which might as well say "couldn't be bothered") or "coming soon".
Best of luck with it!