I would like to do few types of API (HTTPS) calls that takes inputs like Authorization header (or) Username/Password for authentication and gives a json body as output.
Can you please suggest me the correct processor and steps to make this call and generate a flow that will hav the json body as content.
You can use the NiFi GetHTTP processor - https://nifi.apache.org/docs/nifi-docs/components/org.apache.nifi/nifi-standard-nar/1.6.0/org.apache...
This processor allows you to specify dynamic properties which can be used to send headers.
The output of the processor on success can then be routed wherever you would like.
You can see some examples of this processor in use here: https://ddewaele.github.io/http-communication-with-apache-nifi/