$modx->setPlaceholder('msg', 'Hello world!');
[[greeting]] [[+msg]]
In this case, you would need both, otherwise, the placeholder will be cached after the first execution of the Snippet.
Damn it i’v read about this but tryied it like [[!+msg]] not like [[!greeting]], my bad sorry...
[[$message?msg=`Hello world!`]]
<div class="notice">
<p>[[!+msg]]</p>
</div>