I have built a chatbot using AWS Lex and lambda. I have a use case where in a user enters a question (For example: What is the sale of an item in a particular region). I want that once this question is asked, a html form/pop up appears that askes the user to select the value of region and item from dropdown menus and fills the slot of the question with the value selected by the user and then return a response. Can some one guide how can this be achieved? Thanks.
Lex has something called response cards where your can add all the possible values. These are called prompts. The user can simply select his/her choice and the slot gets filled. Lex response cards work in Facebook and slack. 
In case of custom channel, you will have to custom develop the UI components.
Using aws lambda to render an html page in aws lex chatbot