How to add javascript

Hello:
I'm new to qualtrics and I'm trying to figure out how to change the error message "Please answer this question" for those who don't answer a question. I know this has been discussed before, and I did see some java code for doing this - see below. But I'm not sure where to put the code. I went to general then to header, but I as under the impression that there's supposed to be a button that looks like this <> that one uses to add the script. But I don't see it.
Any guidance would be greatly appreciated!

Qualtrics.SurveyEngine.addOnReady(function()
{
jQuery (".ValidationError").html('');
jQuery (".ValidationError").append("Your custom error message");
});