Stored XSS is a type of XSS that stores malicious code on the application. The demo website is affected of it.
#1. Access to the demo website https://demo.usememos.com/
#2. At “Any thoughts…”, write XSS Payload and save it. In this scenario, I used payload: "><img src>
#3. Now, at Search bar, just type "> (or any character in the payload) and the payload will be triggered.