This was one of the easiest XSS's which I have ever found.
To reproduce it:
- Go to the sign up form.
- Create a new user with random values, but type your XSS payload inside of the "Full Name" field.
- Click on the "Create a new free account" button.
- You'll get your XSS alert.
Their security page with my name listed: https://segment.io/security-response
Proof of Concept and T-Shirt picture: