The challenge is about XPath injection.
I basically injected this payload ' or '1'='1, and by this way so all the users. Very similar to SQLi
' or '1'='1
Here you can see the final image using burp suite
The flag is AppSec-IL{!XPaTh_Inj3ct10n@_ForC3}
AppSec-IL{!XPaTh_Inj3ct10n@_ForC3}