If you need to have this HTML on every page, you need to put it into "Master Page" markup. The "Master Page" is root template of the site.
You can do it via Developer Panel (More->Community Manager->Developer Panel).
Brief explanation:
1. Open "Developer Panel"
2. Switch to "Preview" mode. In this mode all your changes are visible only to you and other developers who are in "Preview" mode.
3. Edit master page markup.
4. Save and check result. (Check that it looks correct on all your pages; e.g. there are not style issues etc.)
5. If you are ok with your change, deploy it (click "Deploy" button). After deployment your changes become visible for all users.
Important Note! You need to have "Developer" access for access the "Developer Panel" page.

How can i put free space HTML on any page of my site...

Hello,
The “Developer Access” provides the following main features:
full customization of the site template (this template is applied to all site pages);
full customization of the page structures (columns, zones);
full customization of the modules on the pages;
creation of custom modules on site pages.
In this mode site is very flexible and customizable. Actually you can customize any part of your site as YOU WISH (including creation of unique modules).

Another important feature is support of CSS theames and CSS styling.