The general principle here is that if your field has a placeholder which is descriptive such as "Enter search here" (or just "Search") then it's a bad user expe

The general principle here is that if your field has a placeholder which is descriptive such as "Enter search here" (or just "Search") then it's a bad user experience if submitting the form does a search for the literal text "Enter search here". However, if the placeholder is a valid example such as "Isaac Asimov", then it's a good user experience if submitting the form does a search for that actual text, especially where trying out the search is a primary means of demonstrating the product as is the case here. Another example might be a flight search service where the placeholders for "Departing" and "Arriving" contain "Los Angeles" and "New York" plus valid leave/return dates. If you're trying to show people (who may not be actively looking for a flight at the time) how awesome your service is, clicking on the "Show Flights" button should use those values because the whole point is to demonstrate your service's unique selling proposition. For a login/demo where your goal is to get people to try the demo and convert into a signup, I suspect you will find it more effective to offer trying a demo account via an obvious, in-your-face call to action rather than dual-purposing your customer login form.