How to add a notice navbar/topbar?

Question

How to add a navbar/topbar, like this on this website? " Welcome to the Question2Answer Q&A. There's also a demo if you just want to try it out. "

I want to use this to advertise some websites, to my website

i use classic theme

Answer 1

Visit to - http://yourqasite.com/admin/users as Admin 

Check this option - Notice at top for first time visitors - HTML allowed:

You will see an text box appear below . 

You can write inside that what ever you wish to show to your users who visited the first time . 

Edit - 

if you want to show the message everytime then this is the way - 

Go to - http://yourqasite.com/admin/layout

Check this option - Custom HTML at top of every page:

You will see an text box appear below . 

You can write inside that what ever you wish to show to your users every time . 

Comments

but i want to appear this topbar, every time a user refresh the page

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I have edited my answer