Thx for the screen dump, but it wasn't unclear where you wanted the button
Luckily the picture tells me that you are using TinyMCE, the "Wysiwyg" choice.
In principle it should be as simple as adding "tt" to the theme_advanced_buttons1 line in the file templates_internal/inc_init_tinymce.tpl However, that doesn't work - at least not when I tested. Searching the TinyMCE forum reveals that tt support is, or at least was, an issue - see for example this thread
. You could post on the TinyMCE forum ...