I recommend using https://www.promoter.io/ to measure NPS. It's a brilliant starting point for getting useful feedback too. I've ended up using it for the conference and coworking community that I run as well as for a software product.
Sorry for the shameless plug, but just by coincidence, I have recently started building a simple NPS tool. Currently it's only a landing page, but I'd appreciate anyone interested to subscribe: https://nps.io
I have extensive experience applying the NPS to the hospitality industry, and I can tell you that it's not a marketing gimmick as I have read in some other comments. That's why I decided to roll my own service :-)
Serious question - why does anyone use a 3rd party service to request a 1/10 score from a customer? Kind of feels like it should be table stakes for a business. If they can't do that then wtf can they do?
The difficult design aspects seem to be when to ask and choosing what other supporting information to gather about who they are, where they came from and how they used your product/service - you can't just hook in an API.
