How can I send HTTP request to click button on the page with JMeter. For example button on the page is
<button type="button" id="StartButton" onclick="CreateConversation()">Start chat</button>
HTTP request does not interact with page elements. It sends out HTTP requests to server. You can use it to send the HTTP request that would occur if your button is clicked.
If you're interested in interacting with UI elements, look into the jmeter-webdriver plugin.
Webdriver is very heavy though. Unless you absolutely need to interact with your page elements, it is a better idea to stick with HTTP Request. You can capture the HTTP request generated by the button click using jmeter's recording proxy. See here.