How can we act if user selects a payload? Could you provide an example here

[rsukla-handy]

web_button = elements.WebUrlButton(
   title='Show website',
   url='https://petersapparel.parseapp.com'
)
postback_button = elements.PostbackButton(
   title='Start chatting',
   payload='USER_DEFINED_PAYLOAD'
)
template = templates.ButtonTemplate(
   text='What do you want to do next?',
   buttons=[
       web_button, postback_button
   ]
)
attachment = attachments.TemplateAttachment(template=template)

message = messages.Message(attachment=attachment)
request = messages.MessageRequest(recipient, message)
messenger.send(request)

so if user selects USER_DEFINED_PAYLOAD then how can I execute a function? which parameter i should look for

[rsukla-handy]

if you click or tap the postback element then I am not getting an additional request? Any idea why this is happening?