Yeah I dug around but couldn't find a method that does that? Is it in editengine?

There's no easy way to do it right now. We should probably make it easier, but the Maniphest approach is in the Editor:

// Automatically subscribe the author when they create a task.
if ($this->getIsNewObject()) {
  if ($actor_phid) {
    $results[] = id(new ManiphestTransaction())
      ->setTransactionType(PhabricatorTransactions::TYPE_SUBSCRIBERS)
      ->setNewValue(
        array(
          '+' => array($actor_phid => $actor_phid),
        ));
  }
}

This kind of isn't great because you don't see the [epriestley (Evan Priestley)] token in the input when you create a task, you just get silently auto-subscribed after you hit "Save". I think a better behavior would be to prefill the input so you can immediately take yourself out, but if we do that it conflicts with EditEngine forms that have a default value.

T10222 discusses some ideas sort of in this vein (have these fields merge application defaults with user defaults, maybe) but I'm not sure that's the right fix.

Here, since the only way to add the behavior is a tiny bit sketchy and I'm not sure how valuable it is, I'd probably just not bother. But I haven't though too carefully about use cases or expectations here.