How to pass arbitrary data from client to middleware in Uploadthing?

<script>
const { startUpload } = createUploadThing("pdfUploader", { ... })
</script>

... omited for brevity ...
<Button
    class="bg-blue-600 hover:bg-blue-700 {upload.status === "uploading" || file.file === null ? "" : "cursor-pointer"}"
    disabled={upload.status === "uploading" || file.file === null}
    onclick={async () => {
        if (file.file !== null) {
            await startUpload([file.file], { input: { foo: "bar" } });
        }
    }}
>
    Start upload
</Button>
...

// @lib/server/uploadthing.ts

... omited for brevity ...
const f = createUploadthing();

export const ourFileRouter = {
  pdfUploader: f({
    pdf: {
      maxFileSize: "16MB",
      maxFileCount: 1,
    },
  })
    .middleware(async ({ req, files, input }) => {
  console.log(input)
})