The easiest way is to add them to the login tpl file:
manager/templates/default/security/login.tpl
Make a copy so you can redo your changes when you upgrade MODX.
A more complicated method that will survive upgrades is to return the extra HTML code from either:
OnManagerLoginFormPrerender
or
OnManagerLoginFormRender
IIRC, that goes something like this:
<?php
/* Plugin attached to one of those events */
$extraFields = '
<input type="checkbox" etc.>
<input type="checkbox" etc.>
';
$modx->event->_output = $extraFields;
return;
See the Tpl chunk for the format and classes (look for the remember me checkbox).
If that doesn't work try:
$modx->event->output = $extraFields;
or
Let me know if you use the plugin and what version worked.