Prefill content type form field with alter hook
You need to implement either hook_form_alter() or hook_form_FORM_ID_alter() (there's no hook_alter() which is what your code is implementing at the moment). 

The second is probably preferable as it targets the form you're interested in:

function prefilllink_form_job_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  global $user;
  $form["field_apply"][LANGUAGE_NONE][0]["value"]["#default_value"] = $user-&gt;mail;
}