@islam_zidan - it is not possible today, to get a request attribute from body of a payload. You could look into a method that utilizes the value and create a request attribute there. You can add accessors on a Class object to get to a variable if required.
But as @AntonioSousa said, looks like something is coming soon.
Hi @islam_zidan !
If you have enabled Grail on your tenant you can use Business Event (new powerful "request attribute" working on 100% of trace, without sampling) :
This new feature enabled data capture from the JSON payload! Request and Answer are supported.