If you want to have some text, for example: "Search Site" inside your Squarespace Search field, you can easily do this with the following jQuery code.

Just add the following Script to your "Extra Header Code".

Then just rename "Search Site" to whatever you want it to say. Keep the quotation marks!

jQuery

<script src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>

<script type="text/javascript">
$(function() {
$('.search-form-pt .queryFieldWrapper input').attr({value:"Search Site" });
});
</script>