I was searching something about the "dashboard icon" attribute and found some questions and posts about, but non of them to solve the simple question:

What to enter in the text field from this attribute?

The answer is:

This text "input field" gets part of the code as this <span class="text input"></span>.
You only need to enter the required <icon class name> e.g. "fa fa-file-o" which is the default one for the sitemap tree. In the end C5 will complete the page code by output the sitemap tree it this:<span class="fa fa-file-o"></span> what represents the icon.

The only thing you need to figure out is what is the class name of your icon you like to have.