The Enter text action can be accessed directly from the toolbar, or by dropping a locator in the editor:
In both cases, a submenu allows you to enter the text you want :
A set of additional keys allows you to use special keys when entering text.
You can use the menu allowing you to insert a special value, variable, etc.
Obviously, to see your text appear in the tested application, the interface element on which you perform this action must accept text entries from the keyboard. However, Agilitest will not forbid you to do so on a wrong item, the final operation will always depend on how your application reacts.