Adding a extra field with request payload?

[#1]

Hi, I want to add one extra field to my request payload.

If my payload is:

{
"firstName":"ABC",
"lastName":"XXX"
}

I want to have my payload as like this:

{
"firstName":"ABC",
"lastName":"XXX",
"id":1
}

But, to do this, I want to extract all the variables and then I assign it to my payload.Instead of doing this, is there a easier way?