Autocomplete Field - AJAX Bad Request 400

Hi there. Sorry in advance if I'm missing some key understanding when using this plugin but I'm really not sure why this might not be working!

I'm using the following example froom the documentation on AutoComplete field:
```
$fm = new \Fieldmanager_Autocomplete( array(
'name' => 'Featured Posts',
'datasource' => new \Fieldmanager_Datasource_Post( array(
'query_args' => array(
'post_type' => 'post',
'posts_per_page' => 10,
),
) ),
) );
$fm->add_meta_box( __( 'Featured Posts', 'crowley-v2' ), array( 'page' ) );
```
When I test the field, the AJAX request fails and says "Bad Request". Looking deeper into where exactly in `admin-ajax.php` it's failing - It's failing because there's no actions register to `wp_ajax_fm_post....`

Am I supposed to write my own action for this? I don't quite understand.