There are a few shortcut keys you can use when replying to a ticket:
R — When viewing a ticket, tap the "R" key to bring up the reply window to start replying.
N — When viewing a ticket, tap the "N" key to bring up the note window to create a new note.
ESC — When viewing a ticket, tap the "ESC" key to hide either the reply or note box. "ESC" will also hide the search bar if it's open at the time.
⌘CMD / CTRL + RETURN / ENTER — When you're ready to add a comment or note to a ticket, tap "⌘CMD + RETURN" on a Mac or "CNTRL + ENTER" on a PC to submit the form.