How to programmatically create an entity-reference when submitting a form?

I'm working with the entity reference module. I have two node types (Type A) and (Type B). The basic concept is to let users reference multiple nodes (type a) into another node (type b).

So I created a custom form with a radio field that shows a list of nodes (type b) and attached it to all nodes (type a) via a block. This form is supposed to let users select the current node (type a) and add it as a reference to a node from the list (type b).

Everything is working as expected except the submit function. How can a new reference be created when a users submits the form?

Example of new reference:

• entity_id: $typeb->nid

• revision: $typeb->revision

• target_id: $typea->nid

This is my form code:

    function mymodule_form($form, &$form_state, $op, $edit = array()) {
      $options = mymodule_list_options();
      $form['list'] = array(
        '#type' => 'radios',
        '#required' => TRUE,
        '#entity_type' => 'node',
        '#language' => $langcode,
        '#options' => $options,
      );

      $form['submit'] = array(
        '#type' => 'submit',
        '#value' => t('Add'),
        "#ajax" => array(
        "callback" => "mymodule_form_submit_callback",
        "wrapper" => "inline-messages", "effect" => "fade" ,
        'progress' => array('type' => 'throbber', 'message' => 'Adding to list...'),
        )
      );

      $form["wrapper"] = array("#markup" => "<div id='inline-messages'></div>");

      return $form;
    }
function mymodule_list_options($node) {
      global $user;
      $matches = array();
      $result = db_select('node')->fields('node', array('title'))->condition('uid', $user->uid, '=')->condition('type', 'type_b', '=')->orderBy('nid', 'DESC')->range(0, 50)->execute(); // This creates a list of nodes type_b from the current user.
      foreach ($result as $us) {
      $matches[$us->title] = check_plain($us->title);
    }
    return $matches;
    }

    /**
     * Implements hook_form_submit().
     */
    function mymodule_form_submit_callback($form, &$form_state) {
     global $user;
      $lid = db_select('node')->fields('node', array('nid'))->condition('title', $form_state['values']['list'], '=')->condition('type', 'type_b', '=')->execute()->fetchCol(); //This gives me the selected type_b nid.
      $nid = node_load(arg(1));  

//Im lost here...

    }

^^^ This the output from the block form. Its attached to all nodes type a. The list is composed of nodes type b from the current user.

I couldn't find anything regarding adding references to a entityreference field.

I apologize in advance if you find this question kind of confusing. I tried to explaining it as best as I could.