Entity Reference autocomplete FAPI example.

example.module

$form['product_line_item_ref'] = array(
  '#type' => 'textfield',
  '#title' => t('Product Line Item Reference'),
  '#description' => t('Enter the product line item.'),
  // The last 4 URL parameters are important.
  // 4: Use <NULL> as a placeholder in the URL when we don't have an entity.
  // 3: Bundle, in our case the specific type of commerce line item but for node, that could page, etc.
  // 2: Entity type, commerce line item in our example but node is also a valid type.
  // 1: Field name that is the entityreference field.
  '#autocomplete_path' => 'entityreference/autocompelete/single/commerce_product_line_item_ref/commerce_line_item/related_lines/NULL',
  '#size' => 60,
  '#maxlength' => 255,
  '#required' => FALSE,
);