If URL Contains x Then Do Y in Twig / Craft Template?
If the string you're looking for is part of the URI segment, then something like:

{% if (craft.request.getSegment(3) == 'mySpecialKey') %}
    &lt;meta name="robots" content="noindex, follow"&gt;
{% endif %}


(Adjusting the 3 for whichever part of the segment it's actually in.

Or if it's part of the querystring, then use craft.request.getParam('mySpecialKey').