Custom Validation on Chat Form

Content

Hi, 

What I am trying to achieve without success is the ability to initiate chat only when certain conditions are met.

In particular, I have added a custom required dropdown field in the chat from (chat_launch.php) (the custom field is based on entity Incidents) with values  -- / Yes / No. When the value of this field is No, I would like to show an informative error and stop initiating a chat session (i.e. chat_landing.php). If the value is Yes only then proceed to show the chatting window. 

The idea is to have the customer's explicit consent prior to initiating a chat session (e.g. Agreeing to terms and conditions).