how do i send Custom Variables

So in the Voiceflow document, it says that is possible to send custom variables through the embed, if i understood it correctly. here is the link:

https://developer.voiceflow.com/docs/customization-configuration#customization--configuration

But I'm not sure how I would create the build in Voiceflow.
I know that I have to use javascript function. But I'm unsure how it's possible for it to read it.
can someone provide me with a example or something like that with the javascript.

It return empty, in the first image. in the second image is how i have create the javascript...