Test

<form id="queryForm">

   <label><input type="checkbox" name="category" value="Category:Purp"> Purp</label>

   <label><input type="checkbox" name="category" value="Category:Arch"> Arch</label>

   <label><input type="checkbox" name="category" value="Category:Source"> Source</label>

   <input type="button" onclick="buildQuery()" value="Abfrage erstellen">

</form>