Your listener id (must be unique)
Your listener function
// log a message in the console once a form layer dialog has been opened
JMap.Event.Form.on.layerDialogOpened(
  "custom-form-layer-dialog-opened",
  params => console.info(
    `Layer form dialog opened for layer id="${params.layerId}"`,
    params.attributeForm,
    params.externalForms
  )
)
JMap.Event.Form.on.layerDialogOpened
This event is triggered each time a layer form dialog is opened (attribute and external forms).