How to make a popup that will show up when opening a specific website?

[MalwareDetective]

Hello,
I'm using chrome beta (33.0.1750.46).
I wanted to know if there is an option to make a popup (a white box in the top of the webpage. like in the picture but with my own text) that will show up when I'm opening a specific page? of course it's not for criminal using or something like that, just to make some kind of a note that I want to see everytime I
I'm viewing a specific page.Thank you very much!

(Here is an example for the box I mean)

 

[DaZa9]

This comes from the website itself, I think.

 

[Malware1]

<script type="text/javascript">window.alert("Hi!")</script>

 

[MalwareDetective]

<script type="text/javascript">window.alert("Hi!")</script>

Thanks but it is possible that this is will show up when i'm opening a specific webpage?