Unfortunately the Dialogflow CX direct ‘session parameters' input field are removed in the simulator, since you are already there in the simulator, what you can do is, to the simulator section of your Dialogflow CX agent, go to the chat input and use the “@” command for session parameters.

This will bring you the menu or suggestions, look for the option related to “session parameters”. “Set session” and select it to enter your desired parameter. No external webhook.