This is the full list of 'extended' key names available in tmux 2.2 (ripped from the sourceripped from the source):
/* Function keys. */ F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 IC DC Home End NPage PageDown PgDn PPage PageUp PgUp Tab BTab Space BSpace Enter Escape /* Arrow keys. */ Up Down Left Right /* Numeric keypad. */ KP/ KP* KP- KP7 KP8 KP9 KP+ KP4 KP5 KP6 KP1 KP2 KP3 KPEnter KP0 KP. /* Mouse keys. */ MouseDown1Pane MouseDown1Status MouseDown1Border MouseDown2Pane MouseDown2Status MouseDown2Border MouseDown3Pane MouseDown3Status MouseDown3Border MouseUp1Pane MouseUp1Status MouseUp1Border MouseUp2Pane MouseUp2Status MouseUp2Border MouseUp3Pane MouseUp3Status MouseUp3Border MouseDrag1Pane MouseDrag1Status MouseDrag1Border MouseDrag2Pane MouseDrag2Status MouseDrag2Border MouseDrag3Pane MouseDrag3Status MouseDrag3Border MouseDragEnd1Pane MouseDragEnd1Status MouseDragEnd1Border MouseDragEnd2Pane MouseDragEnd2Status MouseDragEnd2Border MouseDragEnd3Pane MouseDragEnd3Status MouseDragEnd3Border WheelUpPane WheelUpStatus WheelUpBorder WheelDownPane WheelDownStatus WheelDownBorder These can be used in addition to ASCII printable characters set and combined with M-, C-, and S- prefixes.
