When to send it
Net Promoter Score works best as a repeated measure, not a one-off. Send it to customers who have bought from you at least once and had time to use what they bought, then send it again every quarter or half-year. The trend matters more than any single score, so keep the wording identical each time.
Why these questions
The first question is the standard NPS question: how likely is the customer to recommend you to a friend, on a scale from 0 to 10. Keep the wording as it is, so your score can be compared over time and with published benchmarks for your industry. Scores of 9 and 10 are promoters, 7 and 8 are passives, and 0 to 6 are detractors. Your NPS is the percentage of promoters minus the percentage of detractors, and Hearsay works it out for you in analytics.
The number alone tells you very little. The second question changes with the answer. Anyone who scores 8 or lower is asked what would make it a 10, which turns a vague complaint into something you can fix. Fans who score 9 or 10 are asked what they like most, which tells you what to protect and gives you words to use in your marketing.
The survey has two end screens. Detractors see a short note that you read every answer, so they know their feedback went somewhere. Everyone else sees a thank-you. Both show the coupon, because the coupon rewards finishing the survey, never a particular answer.
What to do with the answers
Read the detractor answers first, ideally the same week. Group them into three or four themes and pick one to fix before the next round. If a detractor left a reference such as an order number in the link, reply to them personally: a quick fix often turns a detractor into a repeat customer.
For promoters, look for repeated phrases. Those are the reasons people buy from you, written in your customers' own words.
Variations
For online stores, the NPS for online stores template adds a question about the last order. For software products, the relationship NPS for SaaS template asks about the product and the team separately.