I am trying to make post a message in a circuit conversation through a Circuit Bot via Rest API call. Please help.
You have two options:
Use an incoming webhook. This is very simple and does not require OAuth as the webhook url includes token and the specific conversation. Anyone with this url can post to a conversation. There are both option, posting as yourself, or posting as a bot. If you want to post as a bot, then you first need to create a webhook bot using the "Manage Application > Custom Apps" page. For more information see https://www.circuit.com/unifyportalfaqdetail?articleId=164448 and other webhook articles on these FAQ pages.
Here is s curl example to post a message on an incoming webhook url.
curl https://circuitsandbox.net/rest/webhooks/incoming/9999999-0b95-4088-b272-5bef80f8e68e -H "Content-Type: application/json" -d '{"text":"hello world"}'
Create an actual OAuth 2.0 bot via "Manage Application > Custom Apps" and use the regular REST API (https://circuitsandbox.net/rest/v2/swagger/ui/index.html). There are several REST examples on github. See https://github.com/circuit/circuit-REST-bot for a simple REST bot example.