The source of the promotion has quite evidently come off facebook, hence the discount code.
To reliably check the validity of this code (for example) or any other for that matter for yourself, go to:
https://www.findmypast.co.uk/pay (or if you are on their home page, click on "Subscribe")
On the bottom right of the page is a thing that says:
"Got Discount Code?"If you have a discount code enter it here [Apply]So put in the discount code, press
[Apply], and (if valid) you will see the discount applied.
So in this particular example (if valid), it is applied to the 12 month deal, not the monthly one.
If you are happy with this, then you can press the blue "Subscribe" button under the 12 month deal.
This is not a RootsChat promotion, and the details can't be verified - and I imagine they can quite rightly withdraw the discount code at any time, or restrict it's use.