David Kimura PRO said almost 4 years ago on StimulusJS, Active Storage and DropzoneJS :
dropzone.on("addedfile", file => {
      setTimeout(() => {
        if (file.accepted) {
          const upload = new DirectUpload(file, this.url)
          upload.create((error, blob) => {
            ########## INSERT HERE #########
            console.log(blob)
            ########## INSERT HERE #########
            this.hiddenInput = document.createElement("input")
            this.hiddenInput.type = "hidden"
            this.hiddenInput.name = this.inputTarget.name
            this.hiddenInput.value = blob.signed_id
            this.inputTarget.parentNode.insertBefore(this.hiddenInput, this.inputTarget.nextSibling)
            dropzone.emit("success", file)
            dropzone.emit("complete", file)
          })
        }
      }, 500)
    })
  }