Chat API GET URL - Add custom variables (ie, current page)

Hi,

I am using javascript 'window.open' to simply launch a chat window with the following url:

https://dev.liveperson.net/hc/P29664714/?cmd=file&file=visitorWantsToChat&site=P29664714

I want to capture custom variables (in particular the current page the user is on) to pass to the agent console. I couldn't find documentation about how to specify custom query string parameters an how they should be written. There is this API ref Chat API (REST) Reference about customVariables but that looks like a POST request.

Thanks for your help.