What's the minimum amount of code needed to have a field show up in Manage Fields? I tried copying the functions from the field module in example modules to get a field to show up in Manage Fields without necessarily any functionality

Looking at the answer to this question, only field_create_field() and hook_field_info() should do the trick but I never got that working with the following code: 

function entityreference_add_new_field_info() {
  return array(
       'entityreference_add_link' =&gt; array(
          'label' =&gt; t('Entityreference link'),
          'description' =&gt; t('Entityreference link'),
          'default_widget' =&gt; 'entityreference_link_widget',
          'default_formatter' =&gt; 'entityreference_link_formatter',
      ),
    );  
  field_create_field('entityreference_add_link');
}


Putting field_create_field() outside of hook_field_info() returns 'call to undefined function' error. 

Update: Here's one thing I've tried without success: 

function entityreference_add_new_field_info() {
  return array(
       'entityreference_add_link' =&gt; array(
          'label' =&gt; t('Entityreference link'),
          'description' =&gt; t('Entityreference link'),
          'default_widget' =&gt; 'entityreference_link_widget',
          'default_formatter' =&gt; 'entityreference_link_formatter',
      ),
    ); 
    field_create_field('entityreference_add_link');


$instance = array(
    'field_name' =&gt; 'entityrefernce_add_link',
    'entity_type' =&gt; 'node', 
    'label' =&gt; 'The Link', 
    'bundle' =&gt; 'node', 
  );

field_create_instance($instance);

}
enter code here
You have already created the definition of the field, but you also need to create an actual instance of that field to use it.

$instance = array(
    'field_name' =&gt; $your_field_name,
    'entity_type' =&gt; 'node', 
    'label' =&gt; $your_field_label, 
    'bundle' =&gt; 'node', 
  );

field_create_instance($instance);


That plus what you have above should be the minimum you need to get a field to appear in your management screen.

See the field_create_instance() docs for more info about available options.