Custom Actions Overrides

I would like to be able to override the "To-Hit" and the "Damage Bonus" for custom actions.
For example, I would like to add a simple custom action for my Artificer's Homunculus Servant's Force Strike, but I cannot do it because If I select the "Stat" "INT", it gives me the correct "To-Hit" but the "Damage Bonus" is not correct and you can't use negative numbers in the "Fixed Value" field for that.