How can i change the form default value but not via hook_form_alter
Let assume you want to create a page. Then create button A which redirect user to /node/add/page?button=a and button B redirect to /node/add/page?button=b

Then in your module create hook_form_alter for node form

function MODULE_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form['#node']-&gt;type == 'page') {
    if ($_GET['button'] == 'a') {
      $form['FIELD_NAME']['und'][0]['#default_value'] = 'aaa';
    }
    elseif ($_GET['button'] == 'b') {
      $form['FIELD_NAME']['und'][0]['#default_value'] = 'bbb';
    }
    else {
      $form['FIELD_NAME']['und'][0]['#default_value'] = 'ccc';
    }
  }
}