In Nifi processor "InvokeHttp" invoked by "ExtractText" where do you write body of POSt request

Before posting this question about Apache NiFi InvokeHTTP and ExtractHttp I have gone through all other questions and their answers but I am still not able to write body. My situation is as below:
(1) "GetFile" Processor: gets file which has 2 URLs on separate lines
(2) "SplitText" processor: splits the 2 URLs
(3) "ExtractText" processor: assigns URL to a property I defined called myUrl (which I use in InvokeHttp – Remote URL property)
(4) "InvokeHttp" : I use myUrl in "Remote Url" property. Have set "Send Message Body" as true.

My message body is constant and will not change.
But my question is, where/how do I include the message body. How can I include the message body in ExtractText (in point 3 above).

I know that in "GenerateFLowFile" we can include message body in property "Custom Text" and then link this to "InvokeHttp". But in my case I am using ExtractText to invoke "InvokeHttp"

Please please help!