@bhadraka Check out this flow definition example for how to prepare and test JSON ahead of an invokeHttp post.
Flow Def File:
https://raw.githubusercontent.com/cldr-steven-matison/NiFi-Templates/main/InvokeHttp_Demo_2.json
As far as the post itself goes. I highly recommend that you test API calls with something like Postman so that you can clearly test what headers and such are required so that the API call works. Only then try to make it work with InvokeHttp duplicating all headers. Figuring out how to get the call to work and how to configure InvokeHTTP at same time is not fun.
TIP: in Invoke HTTP click the + to add key value pairs corresponding to your headers or values you need to pass.