icon property in Gen2 (windows, tabs, sections, elements, tags, notifications) takes an image. Gen2 has no icon set you pick by name, so a string like "folder" shows nothing.
What it accepts
A Roblox asset id.
The same id as a string.
A local file, which is what
getcustomasset returns. Used as given, in secure mode too.A Roblox thumbnail. Blocked in secure mode.
nil, 0 or "" means no icon.
Finding icons
Any image uploaded to Roblox works: copy its asset id. For icons that match each other, upload a set from one icon family, such as Lucide. Upload them at 256 by 256 or larger, because a small image looks soft when it is scaled up. To keep your own images out of the asset list, save them withwritefile and pass getcustomasset(path). In secure mode this is the only way your own icons show.
Built-in icons
These are the icons Gen2 draws its own interface with. Their ids work anywhere an icon does.
There is no built-in folder, server or home icon. Find those in an icon pack.