Your interact with target command is found in ISBoxer under Keymaps -> Base Hotkeys. Set that to a particular bind both on ISBoxer and on all 3 of your in-game wow keybinds and it should work.

You can also modify your keybinds in ISBoxer to automatically IWT on individual keys, so it would essentially bypass the need for this individual key but these keybinds should not be shared with ranged abilities (spells outside of cat/bear form).