This also serves as a short lesson in client-side validation/ server-side validation. I could write a lot more on that topic, but not today.
First, go into the Funding and Transfers tab to set up a withdraw and choose "Available Balance":
This is not the most straightforward way of using the website. However, I consider this is a legitimate and useful feature. *If you don't know how to accomplish this, you are not the intended audience here, but a first hint would be for you to use Chrome and access the Developer Tools menu item.
This is not a way of using the site to be a "work-around that is considered confidential and proprietary to Zecco", nor a "reverse engineering, disassembly or decompilation embodying Zecco's confidential information".