I'm building a Web App on the Google Apps Script platform

When I summon JavaScript alerts e.g. window.alert('Thankyou for submitting your details. You will now be taken to the grammar test') it will give me a message at the top of the alert saying randomString scripts.googleusercontent.com says

This looks messy and unprofessional so I would like to get rid of it before giving to my client

How can I override it?