invoke ahah button event from javascript function

I have a field that uses ahah to add multiple values. The only way to have another input field appear is to click the "Add another item" button.

Is there a way to kick off what is needed from a javascript function. Basically, I have a sidebar widget that does a few things...then I want it to add another input field. The event is "mousedown", so .click() won't work in this case. I don't want to make a new hidden form for this.

Thanks.