How to read the value from key value pair in json in the request body

I am trying to set a request transformer plugins using admin API. I want to append a new key to the request API and assign the existing value of another key to this field. Can anyone let me know how this can be done. Sorry Completely new to this

For example:

Original Req body:

“value 1”: “abc”,
“source”: “test”,


I want to add a new key to this json body as “value 3”: {here I want load the value of source during runtime}

how can I achieve this?