how do I use keyboard to enter annotations?

Sometimes when I want to add annotations to a pdf (such as replace text, delete text, etc.) I have to use the mouse to click on the right symbol in the Comments/Annotations box. Other times, however, I can delete text simply by highlighting the text and pressing the backspace key, or replace the text by highlighting and simply typing the new text. What do I need to do to enable the use of the keyboard, such as in the second description, when it's not working?

Kendra Millis