If you don't like Ualaa's way, here's mine: Having Melee Automaticaly IWT

[Ualaa]

With IS Boxer, or the default game, either way it is impossible to send IWT + Your Keybind, on a single press.

Blizzard has given us the option, in their Keyboard API options, to differentiate down/up as two events.
You can send (one of) IWT or your DPS Key (as the only event on key down).
On the second event (key up), you can send the other (IWT or DPS Key).
You could do the same with Macaroon or a couple of other addons, it is not an IS Boxer specific feature.

If you chose to run this as "only execute a step on key release", then it would be two physical key releases, for two actions.
Instead of two physical key state actions (down + up) for two actions.

In IS Boxer, this is a two step keymap. With one event on step 1, and the other on step 2.
You can go with each step on key release (which is the default), or a step on both key down and key up.