Setting a field position doesn't seem to work

When I use the management SDK to set a field position it doesn't work, it does however place the field at the top of the UI.

It may be better to take an apiId instead of a number value or alternatively take an apiId so I can say e.g "Place this field after X field"

```js
...addSimpleField({
  positionAfter: 'example' // String
})
```