Simple key password

Hi Guys,

I am working in the early concept stages of a site for a handyman business. This will be very basic functionality, home page, about, products/services, contacts etc.

But I am also investigating a way to create a testimonial page where we supply the client with a card with a URL to a site page and a key so they can leave comments. In return we will give them a discount of 5% or so.
These comments would then be fed through to social media platforms.

Is there a page plugin that will enable users with a pre-set key-phrase to leave comments as testimonials?

Many thanks