Self-cast button

I would like the option to bind a single key to trip self-casting. By default, Alt+ self-casts. Currently I can't bind a 2 key sequence manually. I cannot rebind this to use a mouse button because I can't manually bind multiple keys, so I can either not use this, use the defaults which I find uncomfortable, or bind a completely separate set of keys. Can I have a single button the when I hold it down lets me self-cast as it's own setting?