There are several ways you could do this.
1) Return the uid value directly from the snippet (let's call it getPostData), and place the snippet call in your hidden field in the chunk like this:
<input type="hidden" name="uid" value="[[!getPostData]]" />
Note the snippet is uncached ([[!
opening tag) otherwise the first form submission will be cached.
2) Place the snippet call in the chunk tag and have the value passed into a placeholder:
[[$myChunk?uid=`[[!getPostData]]`]]
...and in your chunk set an uncached placeholder for 'uid':
<input type="hidden" name="uid" value="[[!+uid]]" />
3) Recommended: Use setPlaceholders()
in your snippet to output content to placeholders anywhere in your page - you can use it to output to multiple placeholders:
<?php
// please sanitise your POST values, this is just an example
$placeholders = array();
$placeholders['uid'] = $_POST['uid'];
$placeholders['email'] = $_POST['email'];
$modx->setPlaceholders($placeholders);
...and your chunk:
<input type="hidden" name="uid" value="[[!+uid]]" />
<input type="email" name="email" value="[[!+email]]" />
Documentation: http://rtfm.modx.com/display/revolution20/modX.setPlaceholders