A barbarian in our party received a homebrew item that allows her to cast a cantrip using Constitution as her spellcasting ability. However, I can't seem to find any option when creating the magic item to edit which spellcasting ability she uses. I can add the spell, but I cannot seem to change the ability. 

Am I missing something, or is it simply impossible to change a spellcasting modifier given the current magic item creation tools? 

Thanks for any help you can provide!

You could only do that with an Action, but Magic Items cannot add actions at this time. The easiest workaround would be to make it as a feat.