0365Connector card: Could not fetch its input elements. (Help!)

Hello, I am using connector card in MS team. That connector card contains a text field and button of HttpPost action.

The problem is, i couldn't fetch the input field value in the body of HttpPost action. Below image( which described in the doc) also doesn't work, instead on clicking the button it gets 400 bad request. 

Can anyone let me know, what i am doing wrong. Kindly help. Thanks in advance.