A simple command injection challenge will be demonstrated in this walkthrough
Here is the challenge page we got after visiting the given link.
I provided random input in this calculator and intercepted the request with BurpSuite
I got one parameter expression taking the values
expression: 8 5 * 6 6
;ls was used to try to inject commands
After forwarding the request above, it has finally been flagged
ctf{*****_***_***_***_*********}