When you use a select box, just in the view.html.php

$model_partner_origin_id = JModel::getInstance('Partners', 'SamplestrackingModel');
$model_partner_origin_id->addGroupBy("a.acronym");
$lists = $model_partner_origin_id->getItems();

as in the default_form.php

<?php
// Partner Origin
$field = $fieldSet;
$field->jdomOptions = array(
'list' => $this->lists
);
?>
<dt><?php echo $field->label; ?></dt>
<dd><?php echo $field->input; ?></dd>

exist any way to declare a default selected index ?
Not in the xml file !!

Do you want to preffil the value, or a simple fallback ?

To set up a fall back, do it in the store() function of your table.
If you want to prefill a value, do it in prepareTable() in the item model, if I remember well.

You can try to set up a default value in the property box and check or not the prefill property. It is not possible for a FK, so try on a string field, and see how it works.