How do I set the format of a dropdown field to allign text at the top of the field instead of the center?

I have an Acrobat 11 form with simple data entry fields. I have, for the first time inserted dropdowns and most of them are single-line fields. However, there are a few examples where my field has to be a larger size to obscure an unwanted background. (The field will set the background colur to white if data is input but I didn't write the script for that.) When a selection is made it appears centred in the field and I need to be alligned to the top.

I presume I can put some script into the Format tab under Custom format script but I don't know enough about Javascript to work out the syntax. I presume it would be a simple line or two of code. Can anyone help?