Post to custom function with REST API

I want to call a custom function in CRM from books via REST API with Api key.

However my POST parameters are ignored.
If I set parameters as GET, parameters are avaialbe, but I have large parameter values and get the error 'Request-URI Too Large'.

Is there any way to pass parameters as POST to the custom function like this?

sPostUrl = "https://www.zohoapis.com/crm/v2/functions/rest_api_func/actions/execute?auth_type=apikey&zapikey=123456sdfsdfsdf8845345sadfswdf";
mRequest = Map();
mRequest.put("user","test");
sResp = postUrl(sPostUrl,mRequest);