With regard to Microsoft Word, I have read in these forums that the spacebar by itself cannot be used to launch a macro.
But, isn't that what autocorrect does? You can add a key board abbreviation like "txt" and its replacment, "replacement text" to the autocorrect dictionary/library. Then, while in the document, typing "txt" and hitting spacebar does the replacement while keeping the usual functionality of the spacebar as you go.
What is going on from a VBA coding perspective?