Re: how to create custom web request which should send json data

Have you tried using the example provided in the function reference?
It's not sending JSON but it can easily be modified to do so.

The JSON payload should be added to the Body argument of the function.

The simplest example would be:

web_custom_request("step_name",

    "Method=POST",

    "URL=http://<url_here>",

    "Body={json_goes_here:1}",

    LAST );