Zoho Creator postUrl "Content Type (415)" Error

I have a simple postUrl script in a Zoho Creator App to receive an authentication key from an internal application. See below:

myHeader=map();
myMap=map();
myHeader.put("Content_Type","application/json");
myHeader.put("Accept", "application/json");
myMap.put("userName"," xxxxx@xxxxxx.com");
myMap.put("apiKey","xxxxxxxxxxxxxxxxxxxxxxxxxxx");
resp = postUrl(" http://test.myeddy.info/habi-ws-ergon/api/v1/token", myMap, myHeader,false);
info resp;

However, It gives back a 415 HTTP error which I believe relates to the content type of the server API.

The following curl command works fine:

curl -i -X POST --header 'Content-Type: application/json' --header 'Accept: application/json' -d '{"userName":" xxxx@xxxxxx.com", "apiKey":"xxxxxxxxxxxxxxxxxxxxxxxxx"}' http://test.myeddy.info/habi-ws-ergon/api/v1/token

Is there something obvious I am missing?

Any help much appreciated

KW