Can't get custom format category to stay selected in Acrobat

I'm trying to make my date text boxes to autotab and limit to one character.

To do this, I've added a javascript:

function tab_next(next_field_name) {

  if (event.willCommit || AFMergeChange(event).length === event.target.charLimit) {

    getField(next_field_name).setFocus();

  }

}

Then I would go to the text field I wanted to edit and go to Properties > Format, I would select custom under the format category. I would then add this script to the Custom Keystroke panel:

tab_next("NextFieldName");
Note that the NextFieldName is replaced with the date field box I needed

The problem happens when I click on OK and my script is not saved over and the format category changed back to None. How can I solve this?