"Query string search" is probably "Query string search".
It is however not clear if "Query" is an action verb. Given the context that this is the label of a form input field, I suppose this is a noun.
As the Form contains several other fields and the form is to complete the parameters needed to perform a query (there's a separate "Submit"/"Search" button for the verb). I supopose that the term "Query" at start is a noun.
But then "string search" is clearly inverted and should be "Search string" (and the "/qqq" doc page should explain better that in that case "Search" is also a noun) or "String to search"
Am I right to think that this field really means "Search string for the query" ? In fact may be you should even remove the word "Query", which is implied by the other form fields (the whole form is to perform a query.
So may be it should just be "Search string".
URL: https://translatewiki.net/wiki/Wikimedia:Toolhub-d23b80-Query string search/fr